I know that going from the MkII to the MkIII was basically a plug-n-play swap. And I did the MkIII to MkIV upgrade, which was definately a plug-n-play swap. So I would assume that it should also be a plug-n-play swap from the MkII to the MkIV. When you do the change, though, you'll need to remove the stand-alone GPS receiver and plug the antenna lead directly into the MkIV nav drive.

The only thing I am not certain of is if the MkIV will operate properly on the 4:3 small screen nav

As for where to get one, eBay is probably your best option. bimmernav.com used to be the source, but he no longer sells them. Here's the only eBay auction I currently see for one (not mine, so do your own research on the seller before buying)

http://cgi.ebay.com/ebaymotors/ws/eB...category=38653

Check Crevier BMW for prices too.