1994 Passat VR6 overheating when idling
 

A friend of mine has a '94 Passat VR6 that overheats when idling for long periods. I'm suspecting that the cooling fans are not kicking on when they're supposed to. I understand that there are two seperate coolant temp sensors/switches on this vehicle. One on the left side of the radiator and one on the thermostat housing. Which one should I be suspicious of first???