You do not go to the battery or anywhere else , as you are simply using the existing circuitry..you are simply jumping the aux fan sw manually, so the circuit thinks the fan is being called for ..that way you do not need any relays of fuses. etc. You are the Switch.
To make sure you have the correct sw, simply jumper the sw and you will see the aux fan come on..then just bring two wires into the cabin to your toggle sw., while leaving the existing wires hooked up. Just tap into them.
It is the sw with the pigtail wire leads :
http://catalog.peachparts.com/ShopByVehicle.epc?q=1988-Mercedes--benz-560sel-Climate--Control&yearid=1988%40%401988&makeid=63%40%40MERCEDES%2DBENZ%40%4063%40%40MERCEDES%2DBENZ%40%40X&modelid=6309%3AMBC%7C1547%3AED%7C10000060%40%40560SEL&catid=242213%40%40Climate+Control&subcatid=242240@@A%2FC+Temp%2E+Switch&mode=PA
Many old Benz guys do this b/c they tow or they know there will come that time when they can stop an over-heat condition manually.
This beats the old "Turn the heater on " trick....