It is essentially an audio isolator (transformer). Install it if you experience hum or interference in the headsets with audio from the Garmin GPS/XM receiver.
It is placed in-line with the audio output jack on the Garmin GPS.
Did the same, used velcro to mount. e-mailed and phoned Van's back then and told them the plans were deficient here but they appear not yet corrected. As I remember, the plans mention it, but do not tell you how to mount it, where to mount it or how to connect it.