94 T100 light problem

One rear light does not work. However the brake light works.(same bulb and unit) Also one of my backup lights don't work(it is the same side as the rear light that doesn't work.) Also all brake lights work. Also my license plate lights don't work.

I checked all the fuses and they are fine. All other lights are fine. I am not sure if it is faulty wiring or what. I just need help diagnosing the problem and figuring out how to fix it. Any help is appreciated.

Have you checked the bulb for the tail light? If it is a dual element bulb, one of elements maybe burned out.
Back up light - Have you checked the bulb? Have you checked the socket for power? Have you made sure the ground connection is good?
License plate lamps - Same questions as the back up lamp.
Have you made any changes such as adding trailer lighting?

I have bought new bulbs for everything. I am not sure how I would test the power for the sockets. The contacts look fine as fair as I know. I think I might take it to the dealership if I can't fix it after like a month or so. Also I have no added any different lights. It is a dual element bulb. Would I be able to use one of those voltmeters for houses on the socket?

Most autoparts stores sell test lights for less than $5-. They look like an ice pick with a wire attached. Connect the wire to a good ground and probe the socket. If the handle lights up, you have power. If your voltmeter can be set to 12VDC, it can be used instead of a test light.

one contact in my dual element socket is not working. The two license lights aren't getting power from as far as I can tell. I am thinking about cutting into the licenceplate lights wires to see if that is actually the case or if it is just the contacts. Anything else I should know before I do anything?