cooling fan switch or relay? 84 190e
 

I've been trying to figure out exactly what turns on the cooling fan clutch. The fan does not come on. I have put 12 volts to it and it does work. Where is the switch? Is there a relay also or does the switch operate the clutch directly. If there is a relay how do I tell if the relay is bad or the switch? Thanks in advance for any advice. MRB