GM-GMC: third brake light outs, gm trucks, gmc sierra

Question
how do i find out whats wrong with third brake light out?  other brake lights work and bulbs on third light tested good.

Answer
well without knowing the year or any info about the vehicle. i can only say that your contact plate has gone bad. this is a common issue on gm trucks suburbans n tahoes. if you do not have a contact plate its the connector. my 99 suburban had the plates my 05 gmc sierra does not so I'm not sure what year they changed.