IAT only monitors the temps inside the intake plenum to help prevent detonation from less dense air.

your ghetto fan switch rig and underhood temps wont affect this in any way. the MIL is indicating a failure of the IAT system, not that your car is actually sucking up hot air. if you were simply sucking up hot air, you wouldnt get a MIL... your car would just simply run less efficently to compensate for hot intake air.

the problem is the sensor or associated wiring... nothing else.