1. On the RCA connector, move the yellow/black stripe wire (mute wire) up 1 position, so that it is sitting next to the red wire after the move
2. Connect the mute wire (from RCA connector), the light-green parking brake wire (from radio power cord) and the black ground wire (from radio power cord) together
3. Either 1 of the above 3 wires can be tapped to connect to the ground wire of the wiring harness for your car.
This link pretty much tells you how to do it.