95 ELDORADO AIR CONDITION STOPPED WITH A LOUD SPARK NOISE UNDER DASH

You need to replace the blower motor, and maybe the ballast resistor also.

Do we assume the engine ran ok in the donor car ?
Were you able to connect all of the sensors on the older engine to the newer wiring ?
Do you know if you are losing fuel or spark when it stops running?

Find failed part... could be fan switch.

If it is affecting the engine performance, it could be a problem with the compressor.

possible cat converter stopped up need back pressure gage to very or drop the exhaust or have someone accelerate the engine and you listen to the exhaust at the tailpipe at rear of caqr if you hear a hissing noise thats a pretty good indicator

Try to remove the belt for the ac and see if you still hear the noise, as it sounds like you may have blown the seal or bearing out of the ac compressor , not sure just a guess.

I have and 2002 with rear air and had the same noise and problem in the back. There is motor in the back that moves a flapper door open and closed to blend the hot and cold. The motor has plastic gears in it and they strip out making a clicking noise when the motor cannot hit the point that the heat control dial is being set too. Probably the same set under the front dash. You will more than likely need to by a new motor assy.

If this noise is a clicking sound and is coming from the front center of the dash, then it may be the blender door motor which will require removal of the dash to replace it. It will keep the door from switching from heat to a/c. Does the odor smell like you have a freon leak in your a/c evaporator core or does it smell like a rat stuck in your heater and a/c ducts?

the dimmer switch would be where I would look first. See if the conditions change after turning the lights brighter and dimmer.

this noise is the air temperature control blend door motor, a very common problem and very hard to repair, he dash must be removed to access the motor..