To answer the OP's question if it's not too late:
I put mine in the lower taillight assembly, upper chamber ...inbetween the top bulb and the vent. At least I think it was a vent, it's a black cap of some sort. Fits in that spot nicely. Sort of a flat area with nothing there. Perfect spot as it has plenty of reflector behind it when you get the lamp installed
If you need to tap the brake light indicator wire it is the blue one coming out of the brake pedal switch. You can also pick this up by tapping the blue wire going to the drivers side taillight. It goes from the taillight up to a blue plug in connector that goes into the harness