I have not put my 37 Ford PU back together again, but soon. I have 2 or 3- 6volt 4 wire heater motors and after I do put mine back together, if you have not found one, I will share with you. However, what you need to be looking for is the correct heater control knob for a 4 wire heater. Because the control knob reverses the motor to defrost. If you don't use the correct control knob, the motor will not work at all. If you have pictures of your heater and post them, I may be able from the pictures to tell you if my motor will work on your heater because they have different length shaft.
If you can find them, I posted pictures of my heater at least 5 months ago and the 4 wire motor. You can look at what I have, and see if you think it might work on your heater.
Jack