As a temporary cure, buy a small cheap vise grip and pinch the hot water rubber hose coming from the engine compartment to the heater to block the flow of hot coolant. It might be advisable not to put back in place the plastic panel on the driver side so that if a quick change in temperature occurs ( eg. driving in a rain storm which would cool the windshield and make it fog up) you can reinstate the flow for defogging purposes . Also make sure that the vise grip does not cut the hose and does NOT interfere with the operation of the gas pedal