Can I use the standard 6 volt Model T wiring harness with a 12 volt battery?
The 6 volt harness is plenty big to handle 12 volts. As the voltage is doubled the amperage is cut in half, so you only need wire about half for the 12 volts. Go ahead no problems on the harness.
Why not? Everyone does. The wires don't reject the extra voltage. They only care about current.
Yes. I know it is a bit counter intuitive but a harness designed for 6 volts is every bit as capable and is usually more capable of handling 12 volts than the harness designed for a 12 volt system.
Think of voltage as electrical pressure. A 6 volt system needs larger gauge wires to be able to carry the same about of current (amps) as a 12 volts system. After conversion to 12 volts, those wires are now more than adequate.
The question is whether the insulation on 6V wiring is good for 12V? Apparently it is since everyone uses it with no problems.
Modern insulation is more than adequate for 12 volts and is usually rated for 300+ volts. I would not trust original cloth insulation. In either case, add a fuse from your battery to to insure that any short will burn the fuse and not the wires.
Great. So my wiring will be OK. Now, how about the use of a 6-volt horn with a 12 volt battery?