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Despite being warned this was rewiring half the car' I did it anyway because I already had a 3 Speed heater assembly in the stash. It was quite simple really.
This is a Feb 3 1965 car. These instructions will not work for a 64 1/2.
What you need:
1. Three speed heater assembly. You can convert a 2 speed by getting the later motor (two leads, no ground wire at the motor) and the resistor and cutting the correct hole in the front of the two speed case to accept the resistor.
2. Three speed fan switch. This is a four position switch. Off, Low, Medium, High as selected by the connection to the resistor assembly.
3. Three Male 1/4 inch (0.250) crimp Faston connectors with the blade completely exposed. Yellow insulator on the crimp end. This set the gauge range for the wire to be used.
4. One Female 1/4 inch (0.250) crimp Faston connector that is completely shielded. I used a right angle one so the wire did not come straight out from the resistor assembly. This was also yellow, though translucent.
5. Approx 3 feet of 20AWG wire. Better to use bigger wire (smaller gauge number) than smaller here. Looks like the motor leads were 22AWG.
6. Some friction tape to neaten up the harness afterward.

These instructions start with the complete unit. Plenty of instructions out there on rebuilding one.
1. Install unit. Plug the motor into the two bullet connectors in the engine bay. Brown is the hot, Yellow is the low side. There will be no ground connection. The ground wire is for a two speed motor because it has two steps in the windings. If you have a ground wire, you have the wrong motor for this mod.
2. Remove the heater controls. I found that removing the ash tray and bracket was all you needed to do to get to the two mounting nuts for the heater control bezel. Later I had the cluster out because I was swapping out the bulbs for LED's which made wiring up the plug a bit easier.
3. Replace the two speed switch (low, off, high) on the control panel with the three speed unit. It has a pigtail that connects to the resistor assembly.
4. On the plug that goes to the old switch you have the two motor connections and the hot wire (switched with the ignition). The center contact is the Hot. This needs to be jumpered to the lead that goes to the Brown Motor lead. You may want to Ohm this out. On mine, the Red wire fed the Brown motor lead. The Orange wire fed the Yellow motor lead. I put a short jumper from Brown to Red on the Switch Plug. For the Orange lead I put a wire to connect this to the resistor assembly.
5. To secure the wires to o the socket I looped a zip tie through the exit wires from the socket (harness side) and just got it started to keep it in place. Nice and loose. Then I ziptied the two wires and the other zip tie together. Then, cinch down the loose zip tie. Easy-peasy.
6. Plug the switch pigtail and the female Faston on to the resistor assembly.
Done.
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