Tx75Pinz

I want to instal a good time radio in one of my Pinzgauers. The Pinz's are 24 volt and the radio is 12 volts. Both of the Pinzes came with a 24v to 12v converter already installed.

My question is not so much where to tap into 12 volts, but I don't want the smoke genie to appear because I messed up on the grounding. Can someone tell what the deal is with the 24v and the 12v grounds? Aren't the both to the chassis? How do I get a seperate 12v ground so I don't fry the radio?

Thanks ahead of time.