Please Help...At My Wits End

Hey Guys. I have a 95 Pontiac Grand Am 2.3 Quad 4. She is overheating and I cannot figure out why. I first changed the thermostat. Then the water pump, which of course was no fun. Then the temp sensor. My fan maotor comes on when I turn defrost on, but if I let it idle and the temp gets to 180, the fan does not come one, and when I turn just heat on, the fan will not come on, only on defrost. Also, after I have draine dthe fluid and refilled it, the lights read LOW COOLANT for like 10 minutes till I keep adding more and it finally gooes out. What is weird though, is that the light for it overheating never comes on, just the temp guage moves to 280. I have the fan relay, but cannot find it under the hood. Could it be this relay, or maybe my fan motor or something I have over looked. Anything you guys could do would be greatly appreciated. Feel free to e-mail me with suggestions. [email protected]
Thanks,
Mike

The relays should be inside a black plastic rectangular box bolted to the center of the firewall.