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The thermostat indirectly controls fan operation. The t-stat's job is to regulate coolant temp and flow by restricting/opening. If the t-stat is stuck wide open, the vehicle will not build proper coolant temperature, and gas mileage and driveability will suffer. It it sis stuck shut, the vehicle will rapidly overheat. If it is slugish to open and close, the engine temp will fluctuate greatly. When coolant temp reaches a certain level, there is a sensor mounted near the thermostat that sends the temp signal to the ECM and tells it to turn on the fans. There are two relays in the underhood relay center that control each fan. One is marked primary and one secondary (or auxillary). Check the relays first if you don't have a check engine light on (since a bad temp sensor should trip a CEL). Try turning the a/c on MAX to see if the fans operate then. The HVAC is set up to turn on the fans automatically, regardless of coolant temp sensor reading, when the A/C is turned to max. If the fans still don't come on, you may have a bad fan motor or two.