I did it with a six volt system . I used booster cables from the solenoid to the 12 volt battery. That way it was isolated and still only six volts to the rest of the electrical system. It worked well. I made the final contact at the 12 volt battery on the floor. A little sparky but it sure made the starter spin and a quick start for the Merc. You can see how it worked in this video.
https://www.youtube.com/watch?v=wO25Y2zgShE