They are almost unobtanium. I haven't found any source for the 2 speed yet and the motor is bout as bad also. The only place I know of is a junkyard. Unless there is a supplier I haven't heard of and that could be.
I checked several places including Concourse and C&G with no luck.
I don't know if a mustang is the same. But I'm kind of thinking if there is it might be the length of the shaft for the knob or something. Like it might need spacers or it might be too short.
There is some pics of the base of 2 dif years here that you could compare the connecting plugs.
Mustang Parts House
I see them for trucks also but not sure what the difference is there either.
Deb