Well I believe the 'Headlights' in use signal wire is what you're talking about.

Its used to tell the RioCar/Empeg to dim the display to the (preset) display brightness value previously selected.

Don't quote me on this, but I would be very surprised if the Headlight on signal wire and the Cellphone Mute signal wire are not using the same RS232 Transceiver chip inside the player (just different pins on the transceiver chip) to convert the incoming higher voltage (ie. the 0 to 12v+ signals to the lower 3.3v signals actually read and handled within the hardware in the Empeg.

If this is the case, then the Transceiver should handle over 12 volts - but how much more I wouldn't care to say [maybe up to 24V but I wouldn't like to test it to find out - could be expensive to get fixed if it didn't].