[supercharged]

Originally Posted by tincan1966

They mainly interrupt the positive circuit because a short to ground to make something work is easier than trying to get alternate power.
Also- old wiring does not drain batteries, just because it is old- defective components drain batteries.

If I were to install something as you are wanting, considering the power to the ignition switch feeds from a wire off the horn relay, I would interrupt the circuit from the horn relay to the "BATT" side of the ignition switch. You can use lower amperage components and not have an obvious interrupt switch. This can be done with a remote wire off the battery post of the starter to a hidden switch- a simple "on-off" to trip a relay.

Just a suggestion.

But what ground?
If the battery is not connected to the chassis and engine block and you are sitting on four rubber insolators, there shoud be no ground.