well have you tried something simple.....

pull the tailight bulb and replace it with a known good one

sounds as if one filament has crossed and is shorting the other

if it works great and you've save a lot of gound wire checking

if not it cost the price of an 1157 two filament bulb (less than 2.00 us) and you may need it alter

Tony H is correct. Your car has a bad ground connection. It will be located somewhere in the ground wiring for the brake light circuit on the driver's side of the vehicle. Trace the wiring from the light assembly back to where it connects to the car body; the problem could be where the wiring is fastened (usually with a ring connector under a bolt or sheet metal screw) to the body itself, or it could be further upstream, in an inline connector or the circuit board, back towards the brake light bulb. It is possible that the bad connection is inside the tailgate; to access, you have to remove the upholstery panel on the inside. The entire ground path must be good electrically: for each connection in the ground circuit, disconnect, clean every surface until bare metal shows, and reassemble.

BTW: any significance to the "5150" part of your screenname? That number has particular resonance vis-a-vis the California Welfare & Institutions Code.
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David Brick, Santa Cruz CA, 1988 245

I don't know where to direct you specifically but it"s definetly a Grounding problem. When you step on the Brake and other lights go out, your electrical path is being interupted.

Sorry I can't help further
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'75 Jeep CJ5 345Hp ChevyPwrd, two motorcycles, '85 Pickup: The '89 Volvo is the newest vehicle I own. it wasn't Volvos safety , it was Longevity that sold me