[ 0024 ] fig1 shows a complete pre - assembled tractor cab unit 10 which can be mounted on and connected to a vehicle body during the final assembly . the tractor cab 10 includes essentially of a four - post rollover frame to whose four corner posts 12 , 14 , 16 enclosure components are fastened , that engage , among other items , the doors and cab windows . the tractor cab 10 has a cab roof 18 and a floor plate ( not shown ). in its rear region a fender 20 is fastened to each side . on the inner side each fender 20 is followed by an air filter housing 22 , of which only the left one is shown . these air filter housings 22 are connected with air channels extending vertically in the region of the rear corner posts 14 , 16 and are accessible from the outside through rectangular covers as was described in ep - a - 0 438 076 . on the rear side of the tractor cab 10 a wall unit 24 is mounted . wall unit 24 is a pre - assembled module which can be formed independent of the remaining tractor cab 10 and which includes a support structure 26 . the support structure 26 is essentially l - shaped and is provided with a cab side wall 28 extending vertically and a cab floor plate 30 extending horizontally formed onto it at the bottom . the support structure 26 has a bulge 32 extending into the interior of the cab that contains a free space for , among other items , a hydraulic pump ( not shown ) mounted on the vehicle differential housing . on each side of the upper region of the cab side wall 28 a wing 34 projects outward . the outside contour of the support structure 26 conforms to an opening in the rear side of the tractor cab 10 in such a way that the side edges of the cab side wall 28 are in contact with the air filter housings 22 , the wings 34 enclose a free space between the air filter housings 22 and the rear window 36 , and the cab floor plate 30 is in contact with the remaining floor plate of the tractor cab 10 and is a part of the cab floor . conventional sealing elements ( not shown ) are used to seal the wall unit 24 against the remaining tractor cab 10 . sliding rails ( not shown ) are provided in the region of the cab floor for engaging skids ( not shown ) positioned on the cab floor plate 30 of the support structure 26 . this permits a simple assembly of the wall unit 24 , that can be set upon the sliding rails with their skids and then slid forward into the tractor cab 10 until the opening in the rear of the cab is closed by the support structure 26 . as best seen in fig2 bordering walls 38 , 40 are formed on the inner side of the wall unit 24 . the walls 38 , 40 project perpendicularly from the cab side wall 28 of the support structure 26 and provide stiffening for the support structure 26 , and border a chamber in which a fuse panel 42 is arranged . a fuse panel 42 is supported in bearings on the bordering walls 40 at the side in its lower region over a horizontally extending hinge axis ( not shown in any further detail ), and may be pivoted forward as a flap from the position shown , so that its upper edge moves forward in the direction of the arrow a and opens an access to the rear side of the panel and to the space behind the fuse panel 42 . on the forward side of the panel and , if necessary , on the rear side as well , fuses , relays and other electrical components are arranged , each of which may be combined into pre - assembled component groups , which are provided with plug - in contacts , in order to be able to plug them into corresponding connectors fastened to the fuse panel 42 . these electrical components are connected over one or more wiring harnesses 44 with one or more connectors . the connectors can be connected with corresponding cab wiring harness connectors , engine wiring harness connectors , gearbox wiring harness connectors or other connectors , in order to establish connections to corresponding electrical component groups of the vehicle . two controllers 48 , 50 are mounted , one above the other , on the inner side of the support structure 26 of the wall unit 24 on both sides alongside the bulge 32 . these may be commercially available electronic control units , which typically have generally slab - shaped metal housings and which can perform different control functions of the vehicle . the electric lines 52 of the controllers 48 , 50 are connected with connectors 54 that are inserted into openings 56 in the cab side wall 28 of the support structure 26 . these connectors 54 can be connected with a corresponding plurality of connectors 58 , which establish connections with the corresponding vehicle component groups ( not shown ). the inner side of the support structure with the fuse panel 42 and the controllers 48 , 50 is covered by an upper enclosure element 59 and a lower enclosure element 60 whose shape and visual appearance conforms to the interior equipment of the cab . the enclosure elements 59 , 60 can be assembled and disassembled in a simple manner and permit rapid access to the fuse panel 42 or the controllers 48 , 50 . the enclosure elements 59 , 60 project to the side beyond the cab side wall 28 of the support structure 26 and extends over ventilation outlet channels ( not shown ) of the air filter housing 22 . in its projecting regions 62 the upper enclosure element 59 contains air flow slots 63 which permit an air flow from the interior of the cab into the air filter housing 22 . on the rear side of the projecting regions 62 , not visible in fig2 circulating air filter elements are fastened over the air flow slots 63 , such as , for example , by an applied adhesive . on the outside of the wall unit 24 , as best seen in fig3 bordering walls 64 are also formed onto the cab side wall 28 of the support structure 26 projecting vertically . walls 64 provide stiffening of the support structure 26 and border upon a chamber 66 . chamber 66 receives two controllers 68 , 70 which are arranged one above the other , and which are electrically connected with one of the aforementioned connectors 54 . the chamber 66 formed by the bordering walls 64 can be closed by a cover 72 , so that the controllers 68 , 70 are protected against environmental hazards . furthermore , the connectors 54 and 58 are protected by covers 74 , of which one is shown , which are fastened above the connectors 54 and 58 . although the invention has been described in terms of only one embodiment , anyone skilled in the art will perceive many varied alternatives , modifications and variations in the light of the above description as well as the drawing , all of which fall under the present invention .