Nissan maxima 00 AC belt squeals for a moment and it goes away. If I turn on AC, lights, defrost, the belt squeals. It does not happen at high speed.

If the blower worked, but couldn't get cold air, there may be a leak of refrigerant in the A/C system. If you're complaint is about the lack fan-forced air; that could be a problem with the fan speed switch or a bunch of other places. Most often however, it is the speed control resistor. Have a look at this thread to learn more about it:

Air conditioner fan switch only works on high Nissan Forums Nissan Forum

the ac compressor has an electric clutch mounted on it , the outside of it the one the belt touches turns with the fan belt and everything else when ever the engine is running , the inside part the one that drives the shaft of the compressor moves only when it receives the electric signal from the dash switch for ac to run (there are a few other switches in the same pathway that could prevent it from receiving power too ) if your fan belt skids or squeals with engine running turn off your defrost/ac switch if it goes away then your ac compressor is most likely bad , if belt just continues to squeal then your belt and/or tensioner needs replaced

if mercury is a ford make there will be plenty of tensioners available at the wreckers /junk yards
remember, that car, is a re-badged Nissan so again there will be parts available from Nissan spares or wreckers /junk yards

a belt is not tight enough.

drive belts.lights increase your alternator's turning resistance a loose belt will slip and squeal

will try to assist you. the loud squealing is most likely a belt thats loose. when you start the engine it squeals for what?30seconds? when you turn the heater on it puts a drag of power from your engine and the engine is trying to compensate. so it [engine] asks for power,puts drain on engine and the loose belt squeals.a loose belt will squeal,simply because it's trying to do it's job.but loosing traction. so it squeals. check for play in belts with engine off of course. grab each belt by hand. if it has more than1/4 inch play or [slack] it needs adjustment,or replacement. hope this helps! thank you for choosing fixya.com we are here for you 24/7 7days per week! good luck!!

The squeal is coming from the alternator belt, When the temp is low, the belt is hard and glazed and lost its ability to create friction.

The squeeling is the belt because the air compresor is likely seizing. At idle it can't turn the compressor, in motionthe compressor probably turns and cools the air. Most air conditioners come on when the defroster is on.

the belt is the most likely culprit could also be the belt tension idler that puts tension to keep the belt adjusted

Seems like a drive belt problem, to confirm pour some water on belts the noise should go away or may get louder for a second. If either happens the belt or belts needs to be adjusted or replaced. Good Luck, If I fixed ya, leave me a FIX-YA comment thanks