Usually it is a bad stop lamp switch. Depending on which brake system you have the switch connector wires should be:

a-WH b-OG c-PU d-BN e-BN f-BK/WH or

a-BK/WH b-BN c-BN d-PU e-OG f-WH.

The stop lamp circuit is OG and WH on both connectors. The TCC brak... Login to read more.