Phil,
If you have checked the circuit, fuses and switches and verify a bad heater motor, you will have to remove the console (both parts), the AC housing and AC blower, then the heater box and controls and finally you will see the heater fan assy screwed to the underside of the dash/firewall. Some have had luck with trying to free up and lube the motor by going through the air intake below the windsheild.
Some have suggested cutting open the firewall but I think they were joking! I went through this operation 14 years ago when I got my '72 250. It's not as hard as it sounds but very time consuming. Get a NEW heater motor because you won't want to do this twice.

Happy Motoring,
Mark

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DrDKW