As an example of troubleshooting with a volt tester. The brake light not working. The brake switch is a normally closed switch. It is held open by the brake pedal arm. It has two terminals. Use an ice pick volt tester for all these tests. It is one handed and no trying to read a meter. The is no reason to "measure " the voltage or amps or ohms. Attach the alligator clip to a good clean chassis ground spot. Then touch the pick to each of the two switch terminals. One should light the tester showing you have voltage and the other should not. If that is the case, prop or hold brake arm in the braking position. Both terminals should show voltage. If that is the case move to rear tail light. Take off lens and remove bulb. With brake still held on, attach alligator to good ground and touch the terminal down inside socket. Should show voltage. If that is the case, take bulb to bench and test it with a household 9V battery. You may see a broken filament before that. Report any test failure to us before doing anything. Your doing this little test will help all of us learn to do testing. So appreciate if you follow the procedure and report. We will discuss the remedy/further tests at that time. Just as an exercise.

I don't know what tail lights your truck ended up with. One light on driver side, light on both sides, single or double filament bulbs..... So locate the brake bulb filament socket terminal for this test.

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn