I'm not familiar with your year, but most likely the blend door that closes off the heater from the rest of the system isn't closing. It could be controlled by a cable or by a vacuum canister. If cable, the cable may have broken, the mounting point for the cable broken at the control or the door or the door could be broken or wedged open. If vacuum canister, a vacuum line may have come off or broken. The canister may have gone bad. The arm that links the canister to the door may have come off, broken or become unhooked at either end. And again the door may be wedged open. I don't think there is a shutoff valve to stop the coolant from flowing into the heater, as I said I'm not familiar with that year, but if it does have one it could have a problem with whatever controls it as well.