Could you not just use a relay and wire direct to the battery with a fuse? That would be the quick and easy way. You can use the same wire to the horn to energize the relay.
But it really sounds like grounding needs to be addressed if your wiring is good. 6v systems will work as good as 12v systems normally.

Last edited by LeeP; 04/08/2018 2:26 PM.
1961 ford f500
1953 3100 five window (https://www.facebook.com/1953-Chevrolet-5-window-1533405913413610/)
1955-2 3200 big window (https://www.facebook.com/Leep3200/)
55yrs as a mechanic and retired. (kind of)