You wrote: "... we did add flow control (CTS/DCD) which are shared with the cellphone mute & lights on sense."

I hope you mean RTS/CTS? DCD (Data Carrier Detect) doesn't sound too useful for a non-modem device like the empeg. Or am I wrong here?

And that "is shared by" confuses me a little. What exactly do you mean? On my car, the 'lights on' sense is a very low impedance line (simply because it's directly connected to the headlights), so you cannot use it as an in- or output for anything else. And the telephone mute is directly connected to two other telephone mute lines (from my head unit and from a separate tuner/amplifier), so if you use that for anything else, these units will get really confused...

Daniel