Re: 12 Volt ground question
Yes you can.
Military vehicles I'm familiar with used to be made with feed and return wires for each circuit. But that's overkill. If you take a look at most commercial electrical install jobs they don't bother sanding back the paint, they just run in a self drilling screws somewhere handy. However, that is not always the most durable and effective way to do it, especially if you are running anything drawing a substantial load. Loose screws cause havoc in the head and the body... If you want to be really careful, run a wire to the frame and use a bolt, though a good bolted connection to the body has never failed me yet.