Hi
not sure if all the B4 back lights are the same , but on mine ,the number plate is lit by the back light bulb & a clear patch in the under side of the tail light plastic.
OK, just looked in the manual , and if im reading the blurry little letters correct its Brown and Black w/White stripe are the light wires.
Also the manual shows every wiring Diag having a licence bulb ( with the same color wires , fed from the back light wires ).
The White w/Black stripe is the brake light wire.
My B4 Jap import , dont have a separate bulb :(
lol