ok so with backwords operation i would expect the problem to be at one of the rear doors. there are two ways for ground to be applied to the light, maybe your operation is correct for whichever way ground is being forced to travel. look at the diagram for a bit and i think you will figure out what im trying to get across.

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1996 s10 zr2 with a 6.5 turbo diesel
1996 c2500 suburban with a 6.5 turbo diesel
1981 300sd with a turbo 617
Monte carlo figure 8 car