Does anyone a site with instructions on how to remove the stock headunit?

Try this thread, found in the Radio & GPS forum. You'll find pictures about half-way down. Although it describes putting the whole thing back together, you should be able to follow the steps in reverse for removal.

Removing '02 Stock Changer (http://www.automotiveforums.com/vbulletin/t22925.html)

good luck,
brad