UPDATE:
So for anyone who is considering this, I may have found out why the '04 model is not listed as an acceptable model for the ISimple ISGM75.
On occasion, the box seems to either interrupt or completely block the signal bus for the system. What this means is that I randomly lose my Tach, traction control, oil temp gauge, gas gauge, etc. In fact, all of the gauges go to 0 with the exception of the speedo. To reset the system I have to pull the car over, shut everything off, and open the driver's side door to cut off power to the radio. Once I close the door and start it up again, everything works great until it randomly goes wonky again. Seems to happen about once every 2 months or so. Definitely random as far as I can tell.
Not a big deal really since I know that everything is ok, but man, the first time it happened it sure freaked me out. It's still worth it to me to have a direct iPod interface without having to change out the factory head unit.
I plan on doing the LED mod to my radio and hvac unit this weekend, so I'll be pulling the radio anyway. I'll check and make sure that everything is connected properly and that I've got the latest firmware on the ISimple box.
Just thought I'd report back on my experience..