Tail lights & sidemarkers are out...brake lights work.

Another thing to check is to actually look at the filaments when you think you have brake lights. If you do not have a ground on the bulb, the current goes through the thick brake filament, but then can't exit on the ground, so it then goes through the thin filament and across to the other side and through that thin filament and out. If you are actually seeing the thin filaments glowing, you have a bad ground at the bulb.