So I recently installed this into my friends 06 Jetta http://sproke.blogspot.com/2011/03/w...-input-to.html and was wondering if i could do this in my 2004 G35 sedan? any advice would be appreciated

If you already have the factory Xm receiver, you can do it pretty much exactly like that, otherwise, just do one of the options from this thread:
https://g35driver.com/forums/audio-v...ory-radio.html

So this wouldn't work with just the wiring harness thats in the trunk?

Nope. The factory radio has to see a compatible adapter attached to switch to that input.