Dumb dash ground question

Took battery from cuda and used it as supply. Found out the one led i couldnt get lit(brakes)was due to its ground is dedicated and contained in the round rally plug when connected. Unplugged round plug and applied +ve and -ve directly and it lights.
Radio, dash lights including signals and hi beams all work.
Gotta double check ashtray/glovebox wiring, the GB light no light....
Next is glovebox light