86 Caprice TAILIGHTS WONT WORK! PLEASE HELP!!

What's up guys (& gals?) This is my first post so bear with me! I have a 86 caprice classic and for some reason the tailights have quit working!! Head, Brake & signal lights work fine but when I hit the switch, nothing. Yes they use to work all the time then every know & then they wouldn't come on but it just kept happening more frequently up till now, where they don't work at all. I took it to a shop and after a week the mechanic said he had no idea (although I think he just gave up) major mods that would maybe infer with this issue is 350 swap-aftermarket stereo system-alarm system. PLEASE HELP WITH ANY INFO!!! AND YES I've CHECKED THE BULBS!! (Lol) maybe light sitch???

How about the fuse? Have you checked that?

could also be a ground issue... my buddy and his dad built up a 56 Bel Air and had the same problem, the tail lights worked, then they would work when they wanted to, then they didnt work at all... it turned out to be a faulty ground

sorry if this comes off as idiotic but tail lights or brakes lights

tail lights maybe that head light switch also

brake lights fuse or brake light switch