The air door is operated by vacuum. You'll have to trace it back under the dash and find where it is chewed or disconnected. As posted above, the hot water to the heater core is controlled by a wire operated valve. The clamp that holds the outer case of the cable may have allowed it to slip out of adjustment.
'99 Coachman Mirada F53 V10
TST monitors, Scangage, AGM house batteries
in the shadow of Latir Peak, NM