Hey, thought I'd share. I've been looking for a good quality aux input connection to the factory NAV system ever since I got my Hummer 3.5 years ago. Every review I read about existing interfaces was poor performance and no one was happy.
I found a link on one of the Hummer forums to this guy that makes custom wiring harnesses for Corvettes and other GM vehicles. I sent him a couple of emails and ended up purchasing his AUX-3 TRK Aux input harness for my H3x.
Got it in the mail the other day and installed it today. Super easy and just plugs in between the XM plug on the back of the radio. Ran the 3.5mm jack to the console bucket and it works perfect.
Plugged in the iPhone and the sound is perfect. No feedback, no hiss, no nothing. Sounds like it should. It piggybacks the XM source and since I get XM/Sirius through my iPhone, I never used the factory XM components in the Hummer.
It was around $100 and worth every penny.
If you're looking for a simple way to plug in your media device, this is the way to go.
Here is a link to the harness. Aux-3 TRK