Radio wiring - what's this

[JonathanR]

Like maths when I see wiring my brain freezes.
Am fitting a new radio.
Previous set up had front speakers and two in rear.
Wiring for rear went down hole by heater pipes along the underside and back up through water pipe hole.
What is the knob for. It seems to have 3 positions.

Also while I at it I also found this tucked away. Connects to a small switch and takes power off the feed to lighter socket but just has the red light. Uses speaker cable. Can't think of purpose.



Thanks
T25 Trident


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[bigherb]

Apart from a "pooh" wiring job, looks like a switch to select the speakers.

[kevtherev]

The red light could be a dummy alarm.