well, i'd just run a straight wire to the battery. I ran my wires through the a rubber boot in the front of my firewall. It runs strait down the steering column. If you had a OBS, u'd have to drill holes in ur firewall, which is annoying as hell.
To ground whatever ur using, drill a hole in some metal, scuff up the paint, and ground ur wire right there. Or if ur against messing paint up for a ground, run a wire back to the battery.