strange problem

O.K. here is a strange problem, I have a 97 cavalier and the problem is that the radiator fan wont kick on at all. I popped the cap on the relay and jumped it with a screwdriver and the fan came on so I know it's getting power, I've already replaced the relay and the coolant temperature sensor but neither one of those fixed the problem. The check engine light is on but the trouble code says there is a malfunction in the radiator fan circuit (as if I didn't already know that) The strangest thing about it all is that if I unplug the map sensor the fan comes on. I have checked the entire wiring harness and there are no broken wires or loose grounds of any kind. If anybody has ever had a similar problem or knows how to fix it PLEASE post a reply, I've talked to alot of people about this and nobody seems to know the answer. I know I could wire it direct but I would like to avoid doing that.