Auto volume - plexed cars adjust the volume on the head unit as you go slower/faster.

I'm afraid I'd have to see a wiring diagram before I could advise on fitment. If auto-electrics isn't something you're confident with your best bet it probably going to be persuading somebody else to be the guinea pig.

Absolute worst case, you'll need to add a CANbus converter. (pretty standard in the audio/aftermarket world) There's probably a vehicle speed pulse available that we can use though.