1990 GMC Sierra - rear brake lights and blinker problems

I just purchased a 1990 GMC Sierra, everything seemed fine EXCEPT rear brake lights, blinkers and interior light won't work. front blinkers work fine. Tail lights and back up lights also work, I have Changed bulbs nothing has helped. does anyone one have any sulutions.

Thanks
Zack

did you check fuses?????

Unplug taillights and use a test light to rule out wiring.
Those taillights tend to have problems with rust, in the boards the lights plug into.