66 c10 heater / fan motor connection

[ray_mcavoy]

You have a bad (high resistance) connection somewhere in the circuit.

With no load (motor unplugged), there is no current flowing through the circuit so that extra resistance doesn't create any voltage drop. But when you reconnect the motor it draws current through that bad connection, which in turn drops all the voltage due to the high resistance.

To track down the problem area, test the voltage at several points along the circuit while it is under load (motor connected). Work your way back from the motor to the switch, back to the fuse box, etc. You'll probably see a few zero (or low) voltage readings, then get to a point where you have full battery voltage (or close to it). Whatever is in between those last two test points is the problem.