I like Joe’s idea. It’s a slick setup. Find a round USB adapter, remove lighter socket, insert adaptor and secure. Use McGyver skills to secure. i.e, a small hose clamp behind the dash should hold it in place.Don’t let it touch any other metal parts. A plus is that it will likely clamp onto the electrical contact. That will provide an opportunity to secure a wire to connect your “hot” negative lead. Connect a wire from the car chassis to the center terminal. Solder the wire or in some adapters, the end unscrews and this would allow insertion of a wire.
Be sure to use an inline fuse(5AMP) directly behind the -12 lead that connects to your -12V source.
Keep in mind that any device connected to a lighter or adapter in this arrangement that has metal parts normally connected to ground in a normal 12V application (+12 and -12 gnd like most cars) will now be a short if it were to touch any part of the cars body. An example would be the metal ring of a 1/8″ audio jack may short out if allowed to touch a metal part of the cars chassis. There’s is a reason for the 5 AMP fuse. And to scare you even more. It may wipe out your devices electronics.
All in all, being aware of these factors should mitigate any issues and you’ll live happily ever after. TT