Rear brake lights on 1998 GMC diesel truck not

I would check the cluster of wires in the back of the truck you might have some wires that have pulled apart or shorted out..

try using a test light to find where power start"s and ends you could have a short in the line itself.
Make sure all the bulbs are good as well as the socket itself Is not corroded ... but with the aid of a test light you should be able to determine this
...{also note that the turn signal hazard switch runs through that as well ,it may be the lever which if tilt wheel is in the vehicle the wires become bent and break from the use of tilt wheel. Again the use of a test light will determine this you'll actually have to remove the lower panel to the steering column and figure out which wire goes to where.