Brad,

Interesting idea; however, simply swap the + 12 and non-chassis ground negative for -12 v. Same goes for +/- 5V (or any other voltage)

Re DC power, I wish that automakers would develop and implement a different interface, as opposed to the cigarette lighter plug. First of all, its too large (I know there is an in-line fuse, but no matter, its diameter is way too big. Secondly, we need more of them (I'm not speaking of large SUVs, etc, where there might be multiple receptacles). If the plus was smaller, more of them would fit in the same space, and a few extra could be spread out around the dash. If youw ant to charge your cell-phone (that's 1), play your MP3 while charging it (that's 2), have a radar detector (that's 3) and a portable GPS (that's 4 - I know they run on batteries, but that would be for handheld usage). You can buy a 2 way cigarette plug spliiter from RS, but that is big and bulky. Plus, I'd like for each receptacle to be separately fused. Thanks!