I use an original iPod Dock.
Seriously, Its the best way. Depending on your car, you can fit them into your car, make them look like factory. I took a USB hub (4-port) and got a 12v car adaptor that would power it. then I use an iPod (3rd Gen, though I guess it'll work with em all) and a NavMan f20 running from it.
Now, I have a Ford Ka (UK version) and the cigarette power thingy sits beneath the rest of the dash with an ash tray. Now I have no need for a cigarette lighter or an ash tray, so I unbolted the whole thing and put it in a cupboard for when I sell the car.
Then I obtained the same item from a scrap yard and stripped away all the plastic housing so I just had a 12v cigarette lighter adaptor hanging loose.
Some bits of hardwood/plywood, fibreglass and carpet later, I have a new centre console bottom, with the NavMan in the bottom, the iPod in the top, and after I broke apart the hub I re-routed one of the USB's to the side for a spare port when I need to plug in a phone charger/other USB item
Cause it all looks factory, Its safe from thieves, and cause the dock keeps the jack connected to the stereo, and the USB connected to the power, I can just drop in and go. My iPod is quieter than the radio too still, but not by a great deal. (not enough to bother me anyways)