The SiriusXM FM Direct Adapter connects any SiriusXM dock and play radio with built in FM modulator to any vehicle radio that has a built in FM radio receiver. So when you remove your vehicle's stock radio (head unit) and replace it with an aftermarket head unit you can plug the FM Direct adapter into the FM antenna port on the replacement head unit and then plug your vehicle's FM antenna into the FM Direct Adapter and then plug the FM Direct adapter into the FM out port on your SiriusXM Dock and Play radio. When installed as outlined above, the SiriusXM FM adapter will allow normal FM reception on your vehicle head unit while the SiriusXM Dock and play radio is turned off. And when you turn your SiriusXM Dock and Play radio on the FM Direct adapter will automatically route the FM stereo modulation from your SiriusXM Dock and Play radio to your vehicle's FM receiver and automatically disconnects the vehicle's FM antenna so that no external broadcasts will cause any interference so you get crystal clear stereo audio from your SiriusXM radio. The SiriusXM Direct adapter automatically reconnects the vehicle's FM antenna to your vehicle's radio as soon as you turn off your Satellite radio. It is important to note that the FM Direct adapter must be used with a SiriusXM radio that has a built in FM modulator. All SiriusXM dock and play radios and the Roady BT have built in FM modulators and these radios all provide the best FM reception when used with the optional SiriusXM FM Direct Adapter.