Remove the negative battery cable.
Remove the serpentine belt, just relieve pressure from the tensioner to remove the belt.
There should be 2 or 3 bolts that hold the alternator to the car. There may be a bracket that goes from the front of the alternator to under the coil packs. Remove this.
Once the alternator is free from the car, remove the wiring harness (use a screwdriver to release the clip). Remove the nut holding the big battery wire to the back of the alternator.
Reverse procedure to reinstall.Change an alternator on a 99' Grand Prix?Go buy a repair manual at the parts store.Change an alternator on a 99' Grand Prix?Birdman
I have included a link to the Pontiac site to show you the engine.
You need to use a wrench to move the tensioner, the small pulley in the center, so you can remove the belt from the alternator, unplug the wires, remove the two bolts, one on either side, install new alt. and connect wiring, replace belt. vua-la all done. Good LuckChange an alternator on a 99' Grand Prix?You could get a chilton manual or talk to a mechanic or go to the parts place where you got the alt and see if they can help you.