Tailight issues

This should be an easy diagnosis. First, be sure the left rear brake and signal light is really full brightness. That can be misleading. If it's hard to tell, remove the left back-up lamp bulb and the brake lamp bulb from the right side. If removing either of those makes the left rear brake light go out, that side has a broken ground wire that we'll have to look at.

If the brake lights seem okay, double-check on the left rear tail lamp socket for 12 volts when the tail lights are turned on. Do this with a test light, not a digital voltmeter. The terminal corresponds to the black / yellow wire in the socket. Once verified there is no 12 volts on that wire, if the tail light is working on the right side, there is very little where the break can be found.

Check if the license lamps are working. If they are not, the best suspect is a corroded splice that feeds both tail lights and both license lights. If the license lamps are working, that splice is still a possible suspect, but the much better one is someone spliced in a trailer wiring harness and used Scotch-Lok connectors. Those are fast, but must never be used outside of the cab because they do not seal out moisture. If you find one of these connectors, the truck's original wire is going to be corroded for as much as a few feet in both directions. It will look dark brown instead of shiny copper-colored. All of that corroded wire must be cut away and replaced. Solder will not adhere to that wire. Work your way back until you find shiny wire. Splice the new piece in, solder both splices, and seal those joints with heat-shrink tubing that has hot-melt glue inside. That will seal out moisture.

You can find that special heat-shrink tubing at most auto parts stores now. If you don't want to buy a whole box of assorted sizes, you can buy individual pieces from the Chrysler dealer's parts department.

Here's a link to an article about using a test light if you need it:

https://www.2carpros.com/articles/how-to-use-a-test-light-circuit-tester

You can try checking for the missing 12 volts with a digital voltmeter, but if you get two different readings, the meter shows 12 volts and the test light shows 0 volts, believe the test light. I'll explain what causes that if we come to that.

I wasn't able to post the wiring diagram, but it wouldn't do much good for this problem. Just follow the left rear wiring harness until you come to the fork where the other half goes to the right side. As I recall, that is usually at the rear of the left frame rail.