2002 SL 2 air conditioner problem
 

My 2002 SL 2 a/c works on defrost, but not on Air conditioner. Mechanic says I need to replace compressor. System blows cool air when I turn on defrost, but hot on A/c

Are you saying that heat or cold comes out of the defroster when you turn it on? If cold comes out then the compressor must be functioning. A problem might be in the controls for the Vents.
From your question am I to assume when you use the A/C no cool air comes from any of the vents, just warm air. If so then it might be possible that the compressor does need to be changed. But before I would change it I would determine if the compressor is actually turned on when the A/C is selected and if it is, what is a pressure reading anywhere? The system might be low on refrigerant.
Your question is too vague to answer with any degree of success.

I agree with Unc. If you would elaborate, maybe someone that knows about a/c can help you. It won't be me, as I don't know much about a/c at all. I do know if it were mine, I'd try a direct wiring to the a/c compressor and see if the clutch is engaging or not. It's always a good idea to get a second and sometimes third opinion.

Dont know how to use this system, but cool sir when on defrost, hot when on a/c
 

System blows cool air in defrost mode, but hot when on A/c

Yep, sounds like the blend door is not operating properly. I don't know if it's vacuum or cable controlled on your car. You'll have to find that out for yourself.

Before you pull the Hvac controls out, do like Rube said. Pop the hood, turn the defrost on, and see in the compressor clutch engages. Then do the same with the ac on, and see what happens.