The 2005 - 2007 Jeep Grand Cherokee is based on Mercedes-Benz's CAN Bus. At this time, there is no OEM hard-wired iPod integration available for the GC, only a crappy FM modulator: http://www.wkjeeps.com/iPod.htm
I really wanted a hard-wired solution that would both charge the iPod and play through the OEM radio. The solution I found, allows me to do this. Unfortunately, it does not display MP3 tag information on the radio, however, it leaves the iPod unlocked, so I can make changes to the song while the iPod is playing. NOTE: This uses the Satellite radio input, so if you have Sirius, you are SOL.
There are 2 pieces of hardware necessary to do this:
AUX input adapter for the OEM Radio:
iPod to RCA adapter with charging
Both pieces are made by a company called PIE Electronics: www.pie.net
I wrote a DIY, and hosted it here. Works great!