I just read you original post again and theres something to be aware of.

while there is a single wire going to your headunit, it could be a simple on/off signal, or it could be a serial data signal.
The first is easy - its either on or off (0 or 12v etc as discussed).

If the wire sends a stream of pulses rather than changes to a on or off state, then you have a serial interface and that gets really really tricky.

I don't know anything much about onstar - heard about it years ago when it was first proposed and thats about it.

You should measure the signal with a digital multi-meter set to the DC volts measurement with the range of maximum of 20 volts or so. [connect the black lead to your cars ground./earth - i.e. a metal part of the car, and connect the red lead to your wire.

Read the voltage - is it 0, or 5 volts, or 12 volts?

Then test your onstar system - does the 'signal' on the head unit wire change state [voltage] and does it stay that way until you're finished with the call?

If so, provided the voltage is not much more than 12 volts [and probably more than about 3 volts] when its in a call then I think its a wire that the Mute input on the RioCar.

If it briefly changes voltage only, then thats a serial data stream of some sort and thats more complicated by a long chalk.

Yes, we could make a device that will 'decode' the signal stream sent to mute [and unmute presumably] the headunit, then turn it into the mute on/off signal to the empeg, but that requires someone to stick a 'scope on the signals and work out the signal format, bit rate etc.
But you'd need to find someone closer to you than I am.