Tom,

This Classic Parts harness is good quality at a good price.

The radio is an accessory that had its own lead (it was not in the original harness). It would be an easy add-on (hot lead and dash lamp lead).

A spotlight was also an add-on accessory. What kind of fan (heater fan)? That was also an add-on accessory.

This will explain how accessories were installed.

I'd stick with 6v unless you need 12v accessories. A 6v radio might be hard to find; but, a 6v to 12v converter strong enough for a non-boomer radio is not expensive.