You can enable the internet tethering on iphone os 3.0 "without" jail-broken it,and it's ridiculously easy.
Here is the link show you how to do it ,I have been using it for a month ,work like charm
http://www.mydigitallife.info/2009/06/2 ... jailbreak/
As for as to make it work with AVIC unit ,it's doable with bluetooth connection ,but that does require some serious programing job on the existing AVIC OS .
But for me ,I don't see any point of doing that since Iphone3G/3GS itsefl is already doing a better job.