I am going nuts here

I have a 1992 Buick Century 3.3 with 321,000 miles. Up till last month it was doing fine. It started overheating. coolant level is good. I changed the coolant temp sensor, but still overheats. Funny thing.. only does it when i am sitting for too long, or after i turn on the the a/c and then turn it off. I noticed that the fan is not coming on. I tested by letting the engine run for about 15 min... then i turned on the a/c... warm cold warm.. no fan. then the temp light came on. I am at my wits end. Someone told me about a cooling fan relay... where could it be. any other ideas... i would appreciate it. :banghead:

if there are little relays behind the right headlight. it would be the second one from the left.

It's the fan motor. Have replaced one in a '90 model with 3.3 several times. The fan should run when the AC is on.