Why does your heater on a 1994 Concord continually and nonstop blow on high?

Typically, it's the thermostat relay...if you take your console apart, and remove the screws holding the heater display, it'll come right out. On the back is a black (or blue) relay. It often rattles loose and loses connectivity or corrodes. If this relay is stuck open (may need to be replaced) it'll cause the heat to blow constantly without responding to the buttons being pressed.