1991 freightliner fld120 Brake light issues

If you have power coming from the switch, then its further down the line and could be anywhere. Start with connectors first.

 

I had similar issues with my 2001, I took it to Freightliner because I didn't have time to mess with it. After they worked their magic, sometimes they would work fine, other times nothing. It ended up being a bad ground near the tail lights.

 

Check if there's power supply to the switch and then step on the brakes and see if there's power flowing through the switch. If yes to both, keep going down the wire until you find the issue.

 

Goes through the turn signal switch. Check that out. If you have a watcher wiggle to switch handle.
If that is OK do electrical checks.

 

Take the top cover off. Remove center defrost vent, remove pannel under it. 5,6 screws.

 

Did yours break, like you try to push in a knob and the the whole thing pushes in lol. I had to replace part of the "frame" behind the dash that holds it all in place. Freightliner can still get the replacement pieces.