Check the upper and lower radiator hoses, if you have not already, and check the hoses going into and out of the heater core, they are on the passenger side of the engine bay.. Also, check the thermostat housing, it ends at the motor side of the upper radiator hose. Have you checked the carpet on the passenger side floor board to see if it is wet? If it is then you have a heater core leaking. I had a similar problem, and it ended up being the upper radiator hose leaking around the thermostat housing, and it was burning off as fast as it was leaking. Hope this helps you some.