auxilliary fan

Where's the thermostat(s) for the auxilliary fan on an '83 500SL? My fan goes on when jumped to the battery, but never goes on in normal operation, and the good ole Texas summer is starting to cause some overheating.

[Deltacom]

Just on top of the water pump, behind the dip stick (red in the photo) yours might not be red.

Thanks, Deltacom. Turns out, though, the one I was after is on the A/C drier, in front of the radiator. (It's A/C pressure- activated.) To replace it requires removing and replacing the freon, according to my tech. Thanks for going to the trouble, though!