UPS's usually don't make good invertors, a lot of the cheaper ones don't dissapate heat well enough to run for long periods of time, usually just the 1/2 hour the battery runs out. I think on the dual battery burbans the vacume ball is under the battery tray. The nice thing about having a second battery is you can run a seperate wire from the battery straight to the mp3player/head unit for better sound isolation, the battery acts as a big filter cap. I plan to add one to my truck soon but I don't have as much room under the hood as you
.