i just installed a pioneer avic-z1 a couple of weeks ago. I thought it was gona be a tough install but not really. There really isnt anything difficult, just time consuming. If you have any electrical experience, soldering, splicing, etc., then youll probly be fine, otherwise go pro.
Like your friend told you, you need an adapter to keep onstar and door/alert chimes. I and others on here have used the Peripheral GMAH24 or GMAH24B depending on if you have the monsoon with amp or the base stereo. And also to keep the chimes there is another module, if you read the description at the link it will tell you which chime module goes along with the adapter. You can take a look at
http://www.cardomain.com/item/PERGMAH24. This adapter connects to the factory harness and allows you to connect the aftermarket nav to it. Its not plug and play, you need to do some soldering or connect wires however you want. There are other brands you can take a look at also.
F5fstop wrote up some great instruction for installing the Z1, but im sure the instalation of the AVN6600 isnt too far off. You can take a look to get an idea what happens. here is the thread
http://www.elcovaforums.com/forums/s...ad.php?t=19267.
By the way AVN6600 is a great nav.