On my 1990 Buick Lesabre it is on the underside of the dash in the middle of the car. Go to you ash tray and look under the dash. There will be a rectangular hole through the plastic and the plug will be in there.
Loosen the tensioner bolt on your 1990 Buick LeSabre alternator. Slide the alternator to the left and remove the alternator belt. Remove the wiring harness from the alternator. Remove the alternator retaining bolts. Reverse the process to install the new alternator.