[Reece]

As noted, it is probably repairable, and worth a try. Remove the switch and the actuator arm that operates the switch. See those little dimples around the perimeter of the back cover? slip a thin blade in there and begin to loosen those dimples and work the cover off. Hook the pot (actually it's a rheostat per the schematic on NostalgiaAir since it only has connections at one end and to the rotor) to an ohmmeter and determine where the jump in resistance occurs. Examine and you will probably see a crack at that point if it's a carbon control. You can get rear window defroster repair paint at an auto parts store and fix the crack. The repair has to be smooth obviously to allow the slider to pass over. Let that dry and then use pencil lead as Sean suggested to scribble over the carbon surface.

It might be a wirewound control; some early ones were in the screen circuit like this. Then look for a break in the wire at the suspect point. This sort of wire isn't easy to solder. You'll have to figure on a mechanical repair of the break. Then lightly sand the slider and the track where it slides.

Let us know how it goes! BTW if you end up having to replace the control it would need to be a linear control, not audio taper, in this circuit.