Step 4: Mount the External Jack in Dash
Now that you have the internal jack mounted & wired, find a nice location in the dash where you will mount the external jack to plug in your MP3 player. Carefully drill a hole in the dash just larger than the jack to mount the external jack.
Use the old headphone cable and cut to the appropriate length to connect the internal and external jacks. Plug the cable into the internal jack, and solder the wire ends to the external jack. Take caution to match the pins (refer to plug layout diagram if needed) This cable serves as the quick-disconnect patch cable, should you need to remove your stereo or take apart your dash in the future. Soldering directly without this patch cable could be a pain later if you ever need to remove the dash or stereo for repairs, etc.
When drilling in the dash, be very careful and know exactly what's behind your drill bit so you don't accidentally drill into something important!
On mine, I had a spacer/junk bin that was right below the radio. This made an excellent location to mount the external jack since it was inconspicuous and also I could wire it up at my workbench instead of in the car. Choose the location you like best to mount the jack. Push the jack through your hole you drilled and use the nut supplied with the jack to screw onto the jack and hold it in place.