I have 8 separate power lines going from the bike to the car -- (1) pink electric trim-up, (2) purple electric trim-down, (3) blue running lights, (4) brown brake lights, (5) green turn signals, (6) black unswitched accessory socket, (7) and (8) blue and red switched accessory sockets. I also have two separate white ground lines--one to the Fuzeblock and one directly to the battery. The power distribution happens behind the seat back as seen here--
Yeah, I know -- it ain't pretty! But separate lines makes it easier to troubleshoot problems, and if one line goes out I have back-ups. Like for instance the running light line is really pretty much the same as the switched accessory lines. This whole area is protected by the seat back, which gets used even when the dog platform is in place.
I installed the Powerlet accessory sockets on a piece of wood glued (yes, glued!) to the seat support bracket. This way I didn't have to drill any more holes in the car, and the bracket provides some protection to the sockets.
I chose this location so that the sockets would not be exposed to the dog when the dog platform is in use, and so they'd be handy to the passenger when the seat is in place. The sockets will be used for heated gear, for a heating mat for the dog, and for re-charging batteries on gadgets.
The biggest battery I take camping with me is the one that powers my CPAP machine. I can sleep three nights on one charge, and then recharge it during the day. It has its own protective case, so I can throw it under the seat and let it ride there while it's charging.