The ’91 and ’92 MJs have a whopping six wires coming off the brake light switch. Some feed info to the ECM for stuff like cruise control and torque converter lockup. The white wire with a pink trace had a ground at all times except when the brake was pressed. So, I wired up a relay like this to get to a 12-volt positive feed for the brake controller. I used a switched 12-volt input because otherwise the relay would be on even when the Jeep was off. This would eventually kill the battery.