Why are Brake lights and tail lights not workingk on 1989 GMC truck?

This can be cause by 3 things. The most common being a blown fuse. If this is not the case, I would check the bulbs. If the bulbs are fine, then you might have a wiring problem. The easiest way to diagnose this, is to turn your lights on and use a multimeter to see if you're getting power to the tail light unit. The flow of power should not jump around, it should be a steady flow of power between 6 to 12 volts. If there's no power going to the unit, then you need to trace down the wiring that runs to them, you should also check to see if anything else has stopped working properly, such as turn signals and emergency flashers.