As mentioned, that's the plug for the buzzer that buzzes when the keys in the ignition and the doors opened. the buzzer itself has a square plastic post on the top that fits in a square hole in a mounting bracket ad is locked in by rotating the buzzer 45 degrees or so. Here's a few pics of where it goes from the Ford wiring diagrams (#13). If the car had the Convience group option, it used a different buzzer with 2 sets of terminals and a second plug on the convience group harness (along with that plug) plugged into it. (The convience group option added a second buzzer that buzzed if you left your headlights on and turned the key off) Hope this helps, John