as required , detailed embodiments of the present invention are disclosed herein ; however , it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms . the figures are not necessarily to scale ; some features may be exaggerated or minimized to show details of particular components . therefore , specific structural and functional details disclosed herein are not to be interpreted as limiting , but merely as a representative basis for teaching one skilled in the art to variously employ the present invention . with reference to fig1 there is shown a motor vehicle 1 having a carrier 8 mounted on a rear end thereof for carrying an object 9 such as a bicycle and a detection system 20 . the detection system 20 comprises an electronic controller 10 , a sensor 2 operably connected to the electronic controller 10 and a human machine interface ( hmi ) 4 including a display screen 5 and operator input means 6 , 7 . the electronic controller 10 is operable to deactivate a rear wiper system 3 in response to the signal or signals received from the sensor 2 . if the signal or signals from the sensor 2 indicate that at least one of the carrier 8 and the object 9 is present , the electronic controller 10 is operable to deactivate the rear wiper system 3 of the motor vehicle 1 to which it is operably connected . in some embodiments the deactivation of the rear wiper system 3 may not occur automatically but only in response to a confirmation that deactivation is required . in such a case a message may be displayed on the screen 5 such as “ obstruction detected . do you wish to switch off rear wiper ”? if the answer is ‘ yes ’ then a user of the motor vehicle 1 would touch / press input 6 and if the answer is ‘ no ’ then the user would touch / press the input 7 . it will be appreciated the inputs 6 and 7 could be buttons , switches , be formed as part of a touch screen device or be integrated into the steering column stalk controls . with particular reference to fig1 the motor vehicle 1 has a rear screen ( not shown ) and the rear wiper system 3 including a wiper arm 3 a to clear water and debris from the rear screen and a motor 3 m to move the wiper arm 3 a . the carrier 8 includes at least one support member ( not shown ) that rests upon the rear screen in a position that obstructs motion of the wiper arm 3 a . the system deactivated by the electronic controller 10 is the rear wiper system 3 and in particular the motor 3 m of the rear wiper system 3 . in one embodiment the rear wiper system 3 is deactivated by using the motor 3 m to return the wiper arm 3 a to a park position and then maintaining it in the park position for at least the current vehicle operating cycle by switching off the motor 3 m . in this case the sensing means 2 may be a current monitoring system associated with the motor 3 m for monitoring the current draw of the motor 3 m . if the current draw exceeds a predefined limit this is used as an indication that the carrier 8 or the bicycle 9 is obstructing the motion of the wiper arm 3 a . the motor 3 m is then reversed until it reaches a park position at which time it is switched off for the current operating cycle . it will be appreciated that in such an embodiment the sensor 2 is formed as part of the wiper system 3 and so box 2 on fig1 would be contained within box 3 on fig1 . it will however be appreciated that instead of measuring current draw of the motor 3 m other sensors such as an ultrasonic sensor or a rear view camera could be used to directly sense the presence of an obstruction to wiper arm movement and in such a case the sensor 2 and the rear wiper system 3 would be a separate entities as indicated by the boxes 2 and 3 on fig1 . in such a case , the rear wiper system 3 could be deactivated before the wiper arm 3 a moves from the park position when an obstruction is detected . that is to say , the wiper arm 3 a is maintained in the park position . as referred to above , a user of the motor vehicle 1 could be given on opportunity via the hmi 4 to allow or prevent deactivation . referring now to fig2 there is shown a first embodiment of a method 100 according to the first aspect of the invention . the method commences at box 110 which is a ‘ key - on ’ event for the motor vehicle 1 . the method then advances to box 115 in which monitoring of the motor vehicle 1 for obstructions such as a fitted carrier 8 or an object 9 being transported by the carrier 8 is performed . as previously referred to this monitoring can be carried out by measuring the current demand of a drive motor in the case of a rear wiper system and / or by the use of other sensors such as ultrasonic sensors or a rear view camera . examples of such sensors are given , for example , in the german patent publication 102008027970 ; us patent publication 2006103727 and canadian patent publication 2684783 . based upon the result of the monitoring step in box 115 a determination is made in box 120 as to whether an obstruction is present . that is to say , if the sensor 2 indicates an obstruction to motion of the wiper arm 3 a is present then the output from box 120 will be ‘ yes ’ and if no obstruction to motion of the wiper arm 3 a is detected then the output from box 120 will be ‘ no ’. dealing firstly with a ‘ no ’ output from box 120 the method then advances to box 160 where it is checked whether a ‘ key - off ’ event has occurred . if a ‘ key - off ’ event is determined to have occurred then the method ends in box 199 otherwise the method returns to box 115 to continue monitoring for obstructions . if the output from box 120 is ‘ yes ’ then the method advances to box 140 where the affected system namely the rear wiper system 3 is deactivated . the method then advances to box 199 where the method ends . referring now to fig3 there is shown a second embodiment of a method 200 according to the invention . the method commences at box 210 which is a ‘ key - on ’ event for the motor vehicle 1 . the method then advances to box 215 in which monitoring of the motor vehicle 1 for obstructions such as a fitted carrier 8 or an object 9 being carried by the carrier 8 is performed . as previously referred to , the monitoring can be carried out by measuring the current demand of a drive motor in the case of a rear wiper system and / or by the use of sensors such as ultrasonic sensors or a rear view camera . based upon the result of the monitoring step in box 215 a determination is made in box 220 as to whether an obstruction is present . that is to say , if the sensor 2 indicates that an obstruction to motion of the wiper arm 3 a is present then the output from box 220 will be ‘ yes ’ and if no obstruction to motion of the wiper arm 3 a is detected then the output from box 220 will be ‘ no ’. dealing firstly with a ‘ no ’ output from box 220 , the method then advances to box 260 where it is checked whether a ‘ key - off ’ event has occurred . if a ‘ key - off ’ event is determined to have occurred then the method ends in box 299 otherwise the method returns to box 215 to continue monitoring for obstructions . if the output from box 220 is ‘ yes ’ then the method advances to box 230 . in box 230 it is determined whether a potentially affected system should be deactivated . this is achieved by sending a message to a user of the motor vehicle via the hmi 4 and acting upon the response of the user . if the user responds that ‘ yes ’ the affected system or systems should be deactivated then the method advances to box 240 where the affected system namely the rear wiper system 3 is deactivated . the method then advances from box 240 to box 299 where it ends . that is to say , once a user has decided to deactivate the rear wiper system 3 it remains deactivated for the current ‘ key - on ’ cycle . referring back to box 230 if the output is ‘ no ’ then the method advances from box 230 to box 299 where it ends . that is to say , once a user has decided not to deactivate the rear wiper system 3 it remains active for the current ‘ key - on ’ cycle . although in the embodiment shown in fig3 once the system is deactivated it remains deactivated for the current key cycle , this need not be the case . for example after a deactivation decision in box 240 there could be a time delay of a few minutes such as ten minutes after which the method advances to box 260 and , provided there is not a ‘ key - off ’ event proceeds from box 260 back to box 215 to recheck whether the obstruction is still present and then proceeds from box 215 as previously described . to avoid annoying a user such as a driver of the motor vehicle 1 , the method could then also include a step in which , if the driver has already agreed to deactivate the rear wiper system 3 more than a predefined number of times , the method will end and the rear wiper system 3 will then remain deactivated for the rest of the current key cycle . as yet a further alternative a system deactivated may remain deactivated beyond the current key cycle so that upon starting a new key cycle it will initially be deactivated until it is confirmed that either there is no longer an obstruction present or until the driver has instructed the system the be activated . it will be appreciated that although the invention has been described with respect to a system in which deactivation includes switching of the motor used to move the wiper arm , the rear wiper system could also be deactivated by interrupting a control flow path between a user operable switch or automatic control device such as a rain sensor or a linkage to a front windshield wiper control and the rear wiper system so that the rear wiper system is unresponsive to a wiper activation command from the switch or the control device . it will be appreciated by those skilled in the art that although the invention has been described by way of example with reference to one or more embodiments it is not limited to the disclosed embodiments and that alternative embodiments could be constructed without departing from the scope of the invention as defined by the appended claims . while exemplary embodiments are described above , it is not intended that these embodiments describe all possible forms of the invention . rather , the words used in the specification are words of description rather than limitation , and it is understood that various changes may be made without departing from the spirit and scope of the invention . additionally , the features of various implementing embodiments may be combined to form further embodiments of the invention .