Find your A/C receiver drier. It's behind the passenger side headlight. It looks like a tube, it's that thing that people look into when they wanna find out if they still have freon in the system.

Once you found it.
There is a switch, or rather two pairs of connectors. One is on the receiver drier itself. That one in my car engages and disengages the compressor depending upon the freon pressure in the car.

The other pair is not a switch for me. It looks more like a pair of connectors. Just make sure that it's not jumpered to come on all the time.

I hope that makes sense.. good luck..
let me see if I can take a picture..

__________________
2008 BMW 335i Coupe