I installed a new deck recently and found this following URL to be very helpful. The thread tells you what the wiring does in the back. i needed the wiring scheme because I had to cut the wires and solder them together with the head units wires. You might find use of this URL like I did.
Wiring diagram on stock head unit
Removal of the stock unit is pretty easy. Take a flat tip screwdriver and wrap a towel around the end. There are 8 clips to the center console plastic part. Just pry that plastic piece off. Then remove 4 screws holding the stock unit. I think this PDF file will show you:
http://www.hparts.com/pdf/21011.pdf
Look at page 2-3 or so and it will show you how to remove the stock unit.