I'm shopping for one now. The ones I saw that were to be hardwired would connect directly to your battery via cables. You could mount it under the rear seat...what I've been thinking of is cutting away some of the metal in front of the rear seat, mounting it so the outlets are fairly flush...you could put velcro or something around the carpet edges so it wouldn't pull away as time passed. My only concern is how warm do these units get?
One of my cohorts has one, we've used it in the 'burb for trips to Atlanta...nothing like setting up a LAN so you can play WarCraft while you buzz down 85.
Oh, make sure you check the specs. Some units say "150 watts" - which is total, for like 50watts/outlet.