This new product looks interesting, wonder if it would avoid any of the interference problems with charging and playing music via the Aux without grounding.. iPodNN | SiK launches Pimp iPod car charger with audio output
belkin has had one of those out for a few years. only difference is no replacable fuse, but ive used one for going on 2 and a half years with no issues. Belkin : Auto Kit for iPod w/ Dock Connector you can find them for ~20 dollars online
another option Kensington LiquidAUX for iPhone and iPod - Apple Store (U.S.) according to this thread the belkin adapter works with iphones after some software update (top is info on a hack, scroll down to the responses) http://forums.macrumors.com/archive/index.php/t-333876.html
