I did a quick search online and it looks like autozone and rockauto carry 4 season heater motor replacement for 66 dodge a 100 van. Take a look and see if the motor looks like a match. I'm sure one of the dodge gurus will chime in. Good luck!
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I did see the 4 seasons motor. It has the same shaft diameter and length. The motor diameter is correct but the stock motor is 3.5" and the 4 seasons is 4.5. This would require I build a spacer between the motor mount plate and the heater box. This would also require relocating the heater box control switches.
I took the one for my r-v to the parts store and we measured all the dimensions and connectors and found one that fits almost perfect. took some book work and research by me and the store I deal with but it worked out. hope you got a friend at a local parts store.