Stupid noob question: does 'grounding the parking and mute wires' mean that you solder (or use a wire tap) a ground wire somewhere along the parking brake wire and mute wire? Do you use a third wire to make the connection between the existing ground and the brake/mute wire?

There are a thousand ways to do it, but you want to end up with the 'mute' and 'parking brake' wires connected to the black 'ground' wire for the AVIC, or connected to the same location in the vehicle as the black 'ground' wire.

Personally, I tie all 3 (mute, PB, Black) and then connect it to the vehicle ground.