I've got a twin one on mine, wired up and 'loose' under the seat. Waterproof lids over the sockets, seems absolutely fine! Obviously not accessible on the move, but if you want it for sat nav, set it up first and if you get a decent length cable, it's fine!
http://pages.ebay.com/link/?nav=item.view&id=331203312598&alt=web This is what I fitted the rubber cap is awesome but it wasn't bogof when I got mine...bastards.