The drivers side rear brake light/turn signal only not working

Sounds like you have 'ground' issues. Apply the brakes using a broom stick or more apropriate tool so that the brake lights are activated. Then go to the brake light that is not bright enough. using a test-light with a sharp pointed probe, poke the ground wire close to the bulb (usually a black wire) make sure the test light is attached to a grounded bolt. if the brake light gets brighter with ground added, you need a new ground wire. Splice it in to the existing wire & attach it to a bolt. Lights should work better with proper ground. (each bulb has a positive post & neg. post, don't add ground to the side with power coming through it from Battery (+)(red light on test light) - probe wires FIRST with test light to find (+) POS. Hot wire, and leave that one alone.

The brake light side of the bulbs is powered by the brake switch and travels thru the turn signal switch.
If the third brake light works, I would guess the problem is in the turn signal switch.

Replace the turn signal switch. Bad contact inside causing a crossing of the circuits. When the turn signal is applied it should open the brake light circuit allowing for just the signal to work. Doesn't sound like the switch is doing it's job.

yes it sounds like the turn signal switch is defective, the brake lights go through that switch as well.

Bad earth connection.Clean or replce wire.

brake switch on the pedal bad.bad relay. do you have a second fuse box if so check it. look at the wiring see if it is frayed or chafed check the connections.check all the fuses not just the ones labeled brakes.switch the relay with one that you know is good like turn signal and see if the turn signal works then check brakes.use a 12 volt test light check the brake switch and probe the wires. hope this helped if not write back and i'll give you more ideas.

if your vehicle lights are working all the fuses will be good. follow the cable from the towing socket on your vehicle , it will most likely go behind one of the rear lights to connect into your cars system,
check your tester where the cables join and you should find the problem. hope this helps.

Rear lights are double filament bulbs one side for the lights other side for the brake/turn signals so it still may be a bulb. Do u have a test light or volt meter you can check to make sure u have power there when brakes or turn signals are activated. Or just try another bulb.

check power on rear light positive terminal.rear light fuses,or ground,..lee

check the wires for a cut or something that may be shorting out the lights