I use a relay and a start (black) and stop (red) buttons.

Run the switched power (should be already fused somewhere) to the start button NO contact (Normally Open) to the relay coil.

Ground the other side of the coil.

Run battery power through a fuse to the relay NO contact.

Run the other side of that contact to your equipment.

This is the part that makes it work. Jumper from the power out to the equipment  then to the stop button NC contact (Normally Closed) then from the other side of the stop button to the coil (same place the start button connected).

The equipment will start only hen key is on.

It will have to be restarted anytime power is lost. If power is lost it will stay off until start button is pushed again.

It will stay on after key is turned off. In normal operation use the stop button.

The equipment will run off direct battery voltage.

Hope I explained it right. Mine works. I got my switches in a hurry at Ace Hardware. I got the relay at NAPA.

Good Luck

Dave