Toyota Repair: Electrical bug, toyota camry, body brace

Question
Hello,

I drive an '89 Toyota Camry, 4-cyl.  It's a great car in a lot of ways, but I don't drive at night.  I have an electrical problem where the tail lights work fine unless the headlights are on.  When the headlights are on, the right tail light works, but the left one goes out when I step on the brake.  This has been an issue for the 3+ years I've owned the vehicle.  I have replaced all of the bulbs with the ones that are intended to be used with this car.  When I got the vehicle, one of the tail lights were not correct.  I have had to re-solder a trace twice in the yellow box in the trunk.  Before I did that, I had no brake lights at all.  I believe it's a tail light relay.  I've heard that the current problem is related to the lamp failure sensor.  I have a diagram to bypass this sensor unit, but I don't know where it is located.  If you know anything about this problem, which I assume is more common that most people realize, please let me know.  If you know where to find that sensor, I'd really appreciate more info on that as well.  Thank you very much.

Answer
It sounds like a ground problem, the most common trouble areas are the ground screws in the bulb holder, the wire harness at the left side on the trunk hinge(the wires stretch and break inside) and the tail lamp failure relay, it is located behind the carpet on a body brace, you can easily remove the cover of the relay and check the circuit board for any burned spots.