why does my left tail light have to be different from the right?

i have a 1993 Olds Cutlass Supreme SL, and the tailights were working great. but now when i hit the brakes, only the right and center lights come on, turn signals both work, hazards only work on the right side, parking lights too, only thing that does not work is the brake light and hazard light on the left side tail light, both bulbs are fine, any ideaS?

check all the wiring and the grounds.
ALso pull out the bulbs and make sure they are not corroded or just dirty and not making good contact.

Could hav a bulb burnt out tooo.

i found the problem, its the turn signal switch, i looked at wiring diagrams and found that there are two wires coming from the brake switch, one goes directly to the high mount stop, the other goes into the turn signal switch, then is routed out to each light. its done so that the turn signals work while u have the brakes one, and etc. i made sure by pressing the brake and playing with the turn signal, if i pull it forward enough, the light works fine. so i need a new switch, about 80 bucks...great