Please list the engine size since they all use different setups. On some of them the air conditioning compressor has its own V-belt. It will run all the time but the compressor won't engage unless you have the air conditioning or the defroster on. The AC runs in defrost mode to dry the air before blowing it on the cold windshield where the humidity would condense and cause fogging. The load of running the compressor can make it squeal if it isn't tight. The slippage can cause a burning smell. More commonly the compressor is locked up or tight. You should see the outer disc rotate when the compressor clutch cycles on.