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It looks like it's more for hardcore car audio types than your standard, all-purpose gadget tweaker, but the Aux-Pod looks like it might be the in-car iPod interface of choice for those doing a custom rig-up. Although it appears to be mostly just a cable, wiring harness, and interface black box (read: no cradle or holder for the iPod itself), the Aux-Pod kit does allow a full integration of the iPod's controls into your cars built-in switches, so you can change tracks (and maybe more) using the toggles on your steering wheel or head unit. It will even charge the iPod when it's plugged in (unplugged, strangely, it does not charge). (Thanks, Michael!)
Read - Product Page [Pac-Audio]