Signal Brake On Left And Reverse Lights

Another idea is to make sure you have the correct turn and brake light bulbs in the rear of the vehicle. Have your auto parts store look up the number or numbers for you. Using the wrong type bulb may cause your problem.

If all else fails, you may need to ground the left socket better. The black wire is usually the ground wire, so scrape off a little of the wire covering and twist a short piece of similar gauge wire over the bare spot that you just made in the wire covering. Wrap that with electrical tape to keep that connection dry and to prevent accidental shorting. Then attach the wire to a screw near the socket or drill a small hole in the metal of the vehicle and install a screw, which you can attach the wire to. Sounds complicated, but it's an easy way to give your left socket a better ground.

You may have to give your reverse lights a better ground as well, so you might have to do the same thing for them as well. 

Here's what your turn signal flasher looks like. There are three different types, so I added a picture of all three to give you a better idea of what it will look like.