Denny,
I think I can answer your question. When the radios first came out, they had a DPDT switch on the control, one for the power and one for the dial light. The first replacement switches were still DPDT. Then eventually the replacement switches only had one pole or SPDT and you would wire it to turn off the power and dial light at the same time. With this arrangement the dial light couldn't be wired through the headlight dimmer.
