3rd brake light on with headlights

I have a 1991 Grand Am that has a third brake light ptroblem. When you turn on the headlights the third brake light comes on. The normal brake lights do not. I removed all of the light bulbs and turned on the lights but the third brake light still comes on. When I step on the brakes with the headlights on,the third brake light gets brighter like a normal brake light. With the headlights on the third brake light is drawing 5volts. I replaced the headlight switch but the light still comes on when you put on the lights. I see no pinched wires that would allow a backfeed. I'm lost. Can you help me?

Len Burdock

You will need to take it to someone that can diagnose and tracing wiring you more than likely have a broken or shorted wire.