Jeep Cherokee a/c question

I'm trying my hand at remote diagnosis here at the moment, as the car in question (92 Cherokee, my wife's car) is about 6000 miles away...

Does anybody know if turning the ventilation to defrost will have the system run the a/c? From what I could establish by working out when the symptoms occur (bad knocking and screeching noise), it looks like the a/c compressor has gone south as the noise never appears with the a/c off and appears to come from the right area of the engine bay.Trouble is, a/c off doesn't work too well in the summer in Las Vegas so Something Must Be Done.

Guess I'll spend part of April getting various issues with said Cherokee addressed, but I'm trying to get a feel for the cost of the parts for this to work out if this is getting too expensive or if we're still anywhere near a sensible cost to keep it on the road.

Yeah, it turns the compressor on when the defroster is used (the A/C dehumidifies the air which clears the inside of the windshield quickly). It's most likely the compressor clutch, but usually if you replace only the clutch the compressor croaks not too long afterwards.

Tell her not to run the defroster or A/C if possible. If she has to use the defroster, see if someone can disconnect the wire for the clutch. Reason: if the clutch or compressor seizes, off comes the serpentine belt meaning no alternator, power steering or water pump. That means roasted 4.0.

Try this place: http://www.discountacparts.com/ I bought a new compressor for the Trooper cheaper than I could buy a reman locally. Not any borderline Chinese crap either, it was the real Made in Japan OE piece.