Most likely the brake light switch for the brake pedal. Do the brake lights stay on permanently or just for a few extra seconds? Either way I think it's the switch.
Funny story, I remember driving my Dad's POS 80 something Lebaron to Toronto once and the cruise would not set. Pissed me off the entire way. When I got there I realized the brake lights were on all the time therefore preventing the cruise from being set. The stupid brake light switch was loose. Rigged it up for the ride home and was a happy camper.