To operate a relay with a signal that switches between open circuit and 0V, you need to give it a (semi)permanent (on whenever you want to be able to use the mute functionality - for instance the ignition signal) +12V signal on one end of the coil and attach the kit at the other end of the coil. When the phone kit goes 0V the relay turns on, with the kit open circuit the relay is off.

With a SPDT (single pole double throw) relay you can then attach the common terminal (30 for a Bosch type relay) to the empeg mute input and put steady +12V and 0V signals on the switching terminals (87 and 87a) as required.

/Michael