I had to chime in on this one -- someone adivsed me about this message.

The fader control you're talking about is a Delco design, the same as used
on just about any GM product during the mid 1960's.  If the car only has a
front speaker, the top 2 pins have a jumper.  If a rear speaker is added,
the jumper is removed and this knob put on, with fader built in.

The trick is, although any Delco fader knob should work, a knob for say, a
Cutlass won't exactly look good in a Studebaker.  For this reason you need
to seek out a fader for the year and model of your car.  The best place to
check would be Studebaker specialty parts shops, who generally stock used
parts or have a junkyard to pick from.  Even more wonderful news; these
faders have a high failure rate, so you'll want to check continuity -- it
seems to me these measure around 100 ohms end to end.

Now -- I hope I'm not adding confusion, but are you sure your car was
offered with this fader control?  Just because the radio has the three pin
connector doesn't necessarily mean it had a knob-fader available.  If I'm
not mistaken, some cars had an external control, or possibly a switch, that
faced downward from the dash for rear speaker control.  I'm sure that, being
a Studebaker newsgroup, some people here may know more about this than
myself, but I do know some of these cars (Avanti maybe?) had a speaker
control that was external, with a white plastic knob that matched the
radio's controls.