1998 C1500 Silverado 3rd Brake light problem

In replacing the bulbs in the 3rd brake light I discovered that the 3rd brake lights come on without the brake we pressed, but go out when I press the brake. The brake tail lights work fine. Looking for some insight into what could be causing this problem? Need help.

You might try swapping out the "Stoplamp Relay" in the Underhood Fuse Block. If that doesn't help, then most likely wiring issues somewhere.