UNKNOWN
The remote control would have been handled by one of Naude's people physically, he would have observed the combi and he would then activate the bomb by means of remote control.  The light sensor was built into a secret compartment or panel which Mackenzie and the ANC used to use to conceal explosives and in so doing cross the border with these explosives in the car.  Now the light sensors would have immediately become operative once the compartments were opened and the bomb would then explode and it would only be somebody who actually knew where the compartment was, who would then activate the light sensor.