I think your stereo is grounded by the negative speaker wire.
if the amplifier section of your stereo it's not running in BTL mode, all
the negative speaker wires are the same ground point. You will try this with
a multimeter working in continuity test mode.
AlgaMAN

"Holophote" escribió en el mensaje
m...
The stereo in my Sable wagon has a audio potentiometer
for controling the volume, with an integrated push-on
push-off power switch. The switch part of the pot has
failed. The volume control and everything else is OK.

I have an extensive background in electronics. I pulled
the stereo to see if I could clean or if needed replace
the volume control/switch. The stereo chassis is spot
welded shut. In other words it was never meant to be
serviced. Rather than junk a perfectly good stereo I
attempted to use an auxiliary on-off switch. There are
two modular plugs with several wires attached to each
plug. The positive or main power wire appeared to be
obvious, at first! It was solid red, and was twice
the diameter of all the other wires, maybe 14 or even
12 gauge.

After disconnecting one of the battery cables I cut the
wire and plugged the connector back into the stereo.
After reconnecting the battery cable and turning the
ignition switch on I was very surprised to see that
the stereo still had power! There was no solid black
or ground wire. I used a snap together connector to
restore the red wire. I tried the same thing with other
wires connected to that module. The stereo simply
would not power down. The only way I could kill the
power was by pulling the modular plug out completely.

I'm really confused. There must be a hot or positive
wire as well a ground or negative wire. The other
modular plug had no effect whether it was in or out.
If this wagon was made in the 60's or 70's and had
a metal dash then the ground could be from the metal
stereo chassis in contact with the steel dashboard.
The dashboard in my 1993 Sable wagon is all plastic
so there has got to be a ground wire. If one of the
wires I cut was the ground wire why did my stereo
continue to receive power?

I've designed and tested the electronics in wide array
of devices. Auto electronics is just plain weird! I'm
glad none of my colleagues were around to observe me
this afternoon. I really felt like a fool!

Holophote