Perhaps you should start by measuring the right value. The Sony stalk interface does not interpret voltage, it interprets resistance. I know the two are inter-related, but I'm just saying, start at the bottom of the right ladder.

Also, I don't know if the Chrysler interface simply changes the voltage or resistance on the wire. It's possible that it's got some sort of pulse code modulation or serial communication happening on the wire, and that it only seems to change the voltage as read by your volt meter.