[meric42]

I did a little research and whilst I have yet to confirm my thoughts, here is what I think should work for a two wire installation. If I get a chance I'll check it out in the next week or so.

Attached is a drawing of the switch.
1. Connect Yellow terminal to chassis/ground
2. Connect the two heater wires to Red and Brown terminals
3. Connect the Black terminal to the ignition switch accessory terminal

This should now give forward and reverse function for the heater motor, bearing in mind the two heater motor wired might need to be swapped so that the rotation direction matches the selected position on the switch.

Thinking about it more as i write, as the field coils in the motor are not now switched relative to the armature in the two wire version, the motor might now only rotate in one direction, regardless of polarity. Also the '41 - '48 heater switch is NOT labelled 'Heat' and 'Defrost' - but it IS labelled 'Direct' and 'Indirect' with a cable operated butterfly for the Defrost option. Therefore I suggest that the direction of the motor rotation may only have a minor effect on the defrost function - watch this space as I am now motivated to check this out hopefully tonight.