Someone replaced a turn signal bulb several years ago on wife'sFirebird and this has been an on going problem, not resolved.
No brake lights, with or without lights on.
No backup lights, with or without lights on.
Turn signals work withOUT lights but not WITH lights on, (sometimes get turn signal with lights on, like today, but 99% they don't)
Have an appointment with auto electrical guy next week but I'm anxious to get to root problems.
Sounds like the ground is missing for the rear lights. Turn on the backup lights and ignition switch, then test for voltage on both terminals for one of the backup light bulbs. If you find voltage on both terminals, there's a break in the ground wire.