My girlfriend drives a 1991 Fleetwood (FWD, 4.9L). Recently the heater quit working; even with the climate control set at 90 degrees all it blows is cold air. The A/C works fine but the heater had been slowly going out last year; I would like to fix it before winter rolls in. Could the heater core just be plugged? If so whats' the best way to flush it? - THANKS!!