Could be a number of things from a broken thermostat to a sticking control valve just in front of the firewall. The control valve regulates the amount of water from the block to the coil in the dash. I would suggest you start the car, make a mental note of position of arm on valve, change heater position and see it it moves. Check fuses and relays. I think vacuum controls the valve. Can't remember as I have not looked at mine in a while.
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