IIRC the Air/Fuel ratio gauge...

the pigtail should be going to the/a 02 (oxygen) sensor.

on the Grand Prix's there are 2 Oxygen sensors.

1. Located directy on the rear header right before where she bolts to the downpipe. (sticks straight up, ontop of pipe) Right next to the rear spark plug towards the drivers side.

2. Located back after the downpipe above the "U bend" adjacent to the catilayc convereter

-KT