[Arran]

I believe that we are discussing two separate factors involved in wire selection, there is current and then there is voltage. Other then the heater wiring all of the circuitry in a radio is low current, some of it is high voltage but low current, some is low current and low voltage. In terms of current considerations the thicker the wire the more current it can handle, car wiring is usually low voltage 6/12 volts, but is often high current lit 10-40 amps, the gauge of the wire isn't usually a problem but the insulation might be.
Regards
Arran