Back left brake light and turn signal not working

You're using a digital voltmeter to find what falsely appears to be 12 volts at the socket. Try using a test light instead, and you'll find 0 volts there. Unlike the meter which just measures the electrical pressure, (similar to measuring water pressure at the turned-off nozzle of a garden hose), the test light requires current flow through its bulb to work. Your circuit has a high resistance someplace that is preventing that current from getting through. This would be similar to standing on the garden hose and blocking it 99 percent closed. You'd still measure full pressure at the nozzle until you opened it and tried to get some water to flow.

When only one side is affected, the two most common places to find that high resistance is a burned contact inside the turn signal switch, and a corroded wire where someone spliced in a trailer wiring harness. That happens most often when someone used a Scotch-Lock connector. Those do not seal out moisture.

I took a guess at a C20 to find the engine size you listed. Here's the wiring diagram for the rear lights. Sorry that they did not include one with the signal switch. I'll try to read this one if you need help with that. As best I can tell, it's the yellow wire that is for the left rear brake and signal part of the bulb.

There's two ways to approach this before replacing the switch as a test. If you already have a replacement switch, unplug the old one under the steering column and plug the new one in. Both brake lights should work now if the switch is the culprit. If you don't have a new switch already, the test would be to jump the yellow wire in that switch's connector to the 12 volt feed wire coming from the brake light switch. That should be a white wire.

I'm going to search at home for a service manual that shows the signal switch, then I can tell you with more confidence which wires to jump.

You can also test for 12 volts on that yellow wire right at the switch's connector. If you have it there with a test light when you press the brake pedal, the switch is okay and we have to look closer at the wiring. Be aware too that just removing a trailer wire isn't the answer. If the splice was soldered and sealed well from moisture, it isn't likely to be the cause of the problem. If Scotch-Lock connectors were used, the truck's original wire would be corroded away. In that case the water wicks through the insulation and corrodes the wires for as much as a couple of feet in both directions. When you strip it and you see the copper wires are dark brown, solder won't adhere to that. You'll typically end up replacing a very long section of that wire.

I'll be here about this time every day. Will wait to hear what you find.

Dandy. One in a row.

Disconnect that plug, then look closer at the two mating terminals. See if there is a way you can stick a pick in along the outside of the female terminal to squeeze it tighter so it makes better contact.