'99 Grand Am GT 2DR Coupe V6 3.4L Ram Air Intake

The turn signals will flash irratic, as in twice the normal speed? That happens when a bulb is burned out. You say that you've replaced the bulbs and the issue is a recurring problem. Since the issue seem to be isolated on the left side of the car only, I'd venture to say the issue is in the tail light assembly. That's where I'd begin to investigate. Chances are, the vehicle was involved in an accident at some point and that was an area affected by damage. I'd remove all the bulbs and check voltage on each socket. If that checks out, I'd remove the entire assembly and inspect all connections and the wiring going to the assembly from the harness. Could be the unit should have been replaced but wasn't. It may be as simple as a ground connection? If this dead ends on you, refer to some one with electrical knowledge. It's electrical but you need to isolate it exactly.