The other thread about heaters is for a 55-2nd truck and I have no first-hand knowledge about that heater or wiring (the information in that thread is very hard to follow).
The Advance-Design heaters had one wire - the hot wire from the switch to the heater blower/motor. There is one power lead into the switch and one lead from the switch to the heater/blower motor. The switch produces different speeds by adjusting the output current that goes to the heater blower/motor. I think the switch can get quite hot because it "dumps" unused current (there is probably a better explanation of why it gets hot).
There is no ground lead on the original switch (into the switch or out of the switch) - the switch itself does not need to be grounded. And, there is no more than than one lead from the original switch to the heater motor/blower.
If you have two leads at your A-D heater/blower motor, either you have a replacement heater motor or someone (over the past 50 years) added a ground wire because they had a bad ground connection between the heater housing and the body.
I would put the 12v-to-6v resistor (the big ceramic-style resistor) in the power lead that goes into the switch (this resistor will get hot no matter where you put it). The motor-control switch also gets hot but I bet it gets less hot if the resistor is before the blower/motor switch)
Here is a diagram of a '51 passenger car - the wiring for the car switch and motor is very similar to the wiring for a truck.
http://chevy.tocmp.com/1951accessories/51aim40.htm
