Step 1: Starting out
The first thing you need to do is find the pin-out diagram to tell which pin handles what function. These things are available all over google for whatever port you might find.
What else do you need: Soldering iron, glue gun, continuity tester, shrink wire, port connector, female a/v jacks
Step 2: Understanding the pin-out
Here I'm going to use the six audio channels, the amplifier remote wire, and mute wire
Right now I'm ignoring the rear view camera and video hookups.
Step 3: The connector
Step 4: Extend the leads
Heat shrink wire is available at Radio Shack and will be somethign you wont do without after the first time you use it. It is a larger diameter tube that slips on over your soldered connection, once in position, hold the soldering iron to it to heat it up and the tube will shrink tightly over the connection to insulate it.
Solder a wire to every pin identified by the pinout diagram. This can get pretty tedious. The end result will be very flimsy and will need to be secured later before you plug it into the back of the stereo.
Step 5: A/V hookup
Step 6: Secure the connection
Make one last check with the continuity tester. Since its impossible to test the positive connections when then unit is off, you can still check the grounds. Finally, to avoid being disapointed after you rip the dashboard off your car to install this, with the harness plugged in make sure there is a continuity between all the negative sides of the a/v jacks to the unit's ground wire.