when the a/c or the defogger is on a squealing noise comes from under the hood. could it be the a/c clutch or the belt tensionner?
squealing belt on 1996 Plymouth Voyager
It could be both, but it sounds like your A/C compressor has seen better days and needs to be replaced, Here is our estimator guide for pricing in your area. http://repairpal.com/estimator?car_type_id=1710