Hey scotty i am working on a 2000 chevy impala the third brake light works but the two outside ones want light up when you mash the brakes but tail lights does work what could cause this

No. 1-1

short in the wiring or ground wiring to the non working ones. If you have too much hassle figuring it out, heck i often just rewire power from the 3rd lights that work to the other ones