Here are pictures for putting an ethernet connection into the car docking sled. Remember to break off the ethernet cable's keeper tab so you can easily remove the player from the dock; even if it's more or less permanently installed in the dash, you'll need to be able to pull the player in and out of the sled as part of any installation, deinstallation, or repair procedures.
One thing: I'm not certain if ethernet is fully functional in DC/CAR mode or not without config.ini tweaking. I know that Hijack HTTP and FTP access are disabled by default in DC/CAR mode, but I'm not certain about emplode access or not.
Something else you can do, which is even fancier, is to rig up an
802.11 access point in the car, so you can update your tunes wirelessly from your laptop. I recommend getting the ethernet cable functioning first, though, then complicating it with the wireless thing later.