I recently dealt with a similar situation. I installed an auxillary battery
and wanted to run the radio, CD and cigarettel lighter off that battery
rather than the starting battery. Since there is no wire between the
auxillary battery and the radio, I ran a 14 gauge wire under the carpet
using an electricians 'fish tape,' a coil of spring steel with a loop at
the end. I ran the wire from the auxillary battery through the battery box
to the van floor and along the carpet seam where it abuts the drivers seat,
and then under the carpet to the area of the radio. Next to the steering
wheel columnn I installed a three pole switch so that I can run the radio,
etc. off either battery.

This approach worked for me. Good luck.