Please read this article:
http://www.eoshd.com/content/7813/how-i-opened-my-5d-mark-iii-and-why-you-have-to-be-crazy-to-do-it
http://www.eoshd.com/content/7813/how-i-opened-my-5d-mark-iii-and-why-you-have-to-be-crazy-to-do-it