Left rear brake light and turn signal not working

Did you use a voltage tester at the applicable fuse and stop lamp circuit? Does the highmount brake light work? For the lower brake lites, the stop and turn use the same bulb filament, just not at the same time. The lower brake lights are wired through the turn signal switch. In the diagram, the brown wire is for tail lamps, the other color is for stop and turn voltage. Black is ground.

Here is a guide that will help

https://www.2carpros.com/articles/no-tail-lights-exterior-running-lights-tail-lights-out

Check out the diagrams (Below) see where you are losing power/ground. Please let us know what happens.