Thanks for your responses Brian.
I bought thte new Aux temp switch and put the wire on it. The fan didn't run. On the old one it runs full speed.
I changed it out in about 5 minutes. The coolant only dripped out and there was no mess. THe only test left is to take it up to 100 C and see if the fan comes on. I'm sure it will.
'85 500SL Euro