Before the days of bluetooth, wired car phone adapters used the Mute wire to tell the audio system to quiet down so that you could take a call. Most modern phones connect via bluetooth and use that instead. The hardware mod involves moving what was used as the Mute connection to a different pin on the radio. This pin, in combination with the parking wire, can be grounded to disable the code that keeps you from mucking with GPS settings/viewing video/etc, while the car is moving.