the left turn signal is flashing rapidly as if there were a bulb blown. but all the bulbs are working properly. The right side works fine.
Also, the brake light in the dash is on and all of the brakes have been replaced and calibrated. Any ideas on these issues?

 

Bad circuit boards in the taillights?

 

circuit boards? can you explain in more detail please. i havent removed the tail lights, but i assumed it had bulbs like the front marker lights.

 

The signal flasher is going for a crap. The flasher is kinda like a circuit breaker. If a bigger load is put on it, it will break the circuit quicker, and most of the time, it's a wire problem. The bulbs act as a resister, and make the flasher heat up to break the circuit, it cools, closes, and the light is on, heats up again, and breaks. If it blinks fast say on the left side, but not the right, recheck bulbs, and sockets on that side, along with grounds. If it blinks fast, so try a different flasher for the normal signals