I was thinking along similar lines recently.

You need four conductors to do ethernet. These conductors should be in two twisted pairs for as much of their lengths as possible. Some kind of a wiring hack might work since the cable runs in the car would be very short and not particularly bandwidth-intensive.

Modifying some of the existing pins (for instance, radio and/or serial lines) to do the ethernet might be feasible, but it would require soldering inside the Empeg itself as well as on the back of the docking sleeve. You would have to choose which pins to use very carefully. Crosstalk and interference will be a bitch with all the other stuff going on back there.

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Tony Fabris