1995 Cadillac Deville Squealing

It is from the engine compartment when running? First of all, how did you determine it was not the belt, and that it is a/c? Does it squeal when the compressor engages, and stops when disengages. Does the a/c work, Is there any smoke, or smell? My guess is the compressor is seizing.

Was the belt replaced to address squealing? A few things come to mind to try and narrow this down. You could have coolant on a pulley, or the belt, and the residue will cause squealing. The belt could be wrong(too long), or the tensioner weak causing a lack of belt tension. You could also have pulleys out of alignment, belt not running true. Try dribbling, or spraying some non-oil based liquid on the belt when running to see if squeal changes. Water alone may make a change. A shot of carb or brake cleaner works well, as it has a fast evaporation rate and leaves little or no residue. You could also have an accessory seizing up. If/when you have the belt removed, start it and see that the squeal is gone. With it shut down, spin all driven accessories(P.S, Idler, Tensioner pulley, Alternator, A/C, Water Pump) and see that they spin freely.