Driver brake/turn light

There are two good suspects. One is the turn signal switch, and the other is a broken wire going to the left brake light bulb. If you see the dash indicator light up for the left turn, or the front left signal light works, those cannot be used for clues. They are fed from different contacts inside the signal switch. The left rear bulb is on its own circuit.

The signal switch is the less-likely suspect. It has three separate contacts for the left rear bulb; one from the brake light switch, one from the hazard flasher part of the switch, and one from the signal part. It is not likely all three are bad. What you do have is a yellow wire coming out of the switch and going to the left rear bulb. That terminal could be overheated/arced and making a poor contact. If you can get to that connector, measure the voltage on that yellow wire when you press the brake pedal. You should get twelve volts.

The better suspect is that yellow wire is broken. This is especially likely if someone previously added a trailer wiring harness and used Scotch-Lok connectors. Those do not seal out moisture and should never be used on a car or truck. It is just a matter of time before the vehicle's wire corrodes away.

Rapid flashing is how electronic flashers respond to not enough current flow, as in a burned out bulb. First compare the left and right rear signal lights. Often there are two bulbs on each side but we neglect to notice only one is working on one side. Next suspect would be the wrong bulb. You can switch them between the left and right sides, then see if the problem moves to the right side. A new-style LED will not draw enough current.

A bad ground will cause this too. The clue is the left bulb will go out when you remove the bulb from the right side.