Stop Lamp and Turn Signals Not Working..puzzled

I've read similar posts regarding this issue but none quite like this one. My front turn signals and headlamps work fine. My rear stop lamps, backup lights and turn signals do not. Even the third break light on the cab doesn't work. But the rear running lamps do work.

I do have the 'fast flash' inside the cabs gauge panel which I understand usually indicates a bulb failure. This has been going on for quite a long while and I was ignorant of the significance and really thought nothing of it. It wasn't until I was making a turn one night and got a good hollering from a guy that was right behind me. I checked the fuses inside the cab and under the hood. They look fine to me. I even tried the WD40 trick on the emergency flasher inside the cab. Just sprayed a shot down the switch into the steering column. Didn't seem wise to me but apparently it did the trick for one fella but not me.

I haven't checked the bulbs yet. Requires a polydrive screwdriver apparently. Going to have to get one of those but hoped you men might have other suggestions to figure this out?

Trailer harness gets a lot of **** built up inside where it connects to the taillight harness so that's possible. I fought that for 2 years till i eventually just changed the whole harness