My Pioneer unit has the same feature. It will either mute the audio or attenuate it (your choice) when the wire is grounded. I got tricky and hid a small tactile push button switch just under the shift boot of the stick shift. It's completely hidden from view. Anytime I want to quiet things down for a second, like to listen to the revs or even to place an order at a drive through, I just lay my finger on the switch and its quiet until I let off again.