From what I've read and heard from other members, you can do it either way, but using the AV parts would probably be easier, since the switch for the EXT uses up a lot of space and the AV parts are compatible. You can do a search on the site for them, there have been a couple of different topics and on the other club site: http://www.clubavalanche.com/forums/index.php?
I believe that there is a group buy for the pwr folding/turn signal mirrors. It is a pretty big install though, and it is best if you can find a dealer to do it.
Good luck.