I got my 6-12V conversion from Genessee Products.
They can provide you with just the harness, or the whole works.
I converted from 6V pos grd to 12 neg grd with alternator.
I bought the harness from Genessee, got the 10SI alternator from Autozone and fabricated my bracket. They were great to work with. The wiring on my D-14 was not original. The key switch had been substituted for a different style. They even sent an extra hookup wire and told me how to wire it with my switch.