The only way you can have a wired connection to your factory head unit is if it has an auxiliary port. I'm not sure if the 2004 Rockford Fosgate has one or not. I know my 2001 did not. If you do have one you could connect it that way and charge it with a charger in the cigarette lighter.
The only other option you have with the factory would be a radio transmittor.
I have had an Alpine IDA-X001 for a few years now, they don't make it anymore but it's the earlier version of the current Alpine iDA-X303. I love it because you can connect your iPod to it through a USB. This allows you to control the iPod on the head unit, charges it, and gives you the best quality possible.
Theres a few other Alpine units that do this, but I would make sure to get the ones that have a USB connection because it's a lot faster than the "Alpine iPod cable" and this means you can just connect flash drives to it as well.
I was able to install mine all myself so that didn't cost anything, and it's not hard to do. If you order it from Crutchfield they send you instructions on how to install specific to your car.
I know you said you weren't really looking to get a new head unit, unfortunately if you want a better way to connect your iPod, a new head unit might be the only way.