there are two relays in the underhood relay center (front passenger side) that run the two engine cooling fans. They are the same relay number and can be switched to see if one or the other is bad. The CTS (coolant temp sensor) for the computer also signals the fans to kick on, so if no CEL (check engine light) is on, the sensor is probably good. The factory turn on temperature for fan #1 is about 230 deg f, (112 deg c), so it can get hot before the fans will kick on. The heater control unit should energize the aux fan relay when the switch is set to max, automatically turning on the #2 fan independent of engine temp. I would start by checking the fan relays.