Basics in the video on post #5 here, stock removal is at about 1:40:
http://www.ft86club.com/forums/showt...ight=OEM+Audio
Same video on post #1 here:
http://www.ft86club.com/forums/showthread.php?t=27312
Details are a different story, the video is for a plug-and-play kit. The wiring will likely be the difficult part unless you could find an aftermarket harness. Also, I understand that the opening is slightly larger than a standard double-DIN, so this could cause issues without a good fit kit.
(Or maybe your new aftermarket head unit exactly fits the stock system's wiring harness and mounting hole
.)