(This text from manufacturers brochure)

The Wiz is an effective 2-way radio voice-activated left front speaker switch and left front stereo output mute device that allows seamless and fully automatic interaction between the vehicle stereo and a scanner or other radio receiving device equipped with a mono 1/8” (3.5mm) speaker output jack such as on a GPS, CB, FRS, GMRS, VHF, UHF, Ham Radio or Radar Detector.

The Size is about the dimensions of a deck of playing cards 3”x2”x 0.9” in an encapsulated enclosure with a 90 degree mono 3.5mm plug on a 12 foot cord. The best placement of the WIZ is between the stereo and speaker and is mounted under or behind the dash out of sight.

How it works:  When voice is present (volume, set at a pleasant listening level) on a 2 way-radio, the left front stereo output will automatically mute and the left front speaker will switch to 2 way radio voice thus acting as a high-quality external speaker. All other speakers remain unchanged playing and connected to the stereo. The left front speaker remains switched for 3 seconds after the last voice message is received, then returns back automatically to normal stereo play.