First, test the power port with a cellphone charger or some other device that plugs into the cigarette lighter port. If it seems to provide a solid 12v ... the next thing to do is close the ground loop that exists between the radio wiring and the power port. (trust me, it exists) You do this by making the ground for both devices common, meaning tie them together and then re-ground to frame metal. This should clear up electrical whine or static as well as solidify the electrical circuit between your iPod when on the charger, and your line-in. The Belkin unit that you got is the one with the line-out port on the side of the 12v plug, and it goes to an iPod dock connector, I assume? Feel free to reply or email me.
12v installer and Macintosh/iPod user - Apple Certified Technical Consultant
McIntosh certified 110v and 12v
Sony Mobile ES certified
02 WRX Sedan - $10,000 in audio system including iPod photo 40gig.