The 40-EU30 antenna adapter kit contains both parts (40-EU10 & 40-EU20) to allow for installation of a FM Modulator. These adapters are needed because the FM modulators use standard male/female Motorola connections for their inputs and outputs. Plug the FM antenna plug into the female end of the 40-EU10 which will then plug into your FM modulator. The output of the modulator plugs into the 40-EU20, which then plugs directly into your Factory radio. Check the our compatible vehicles tab to see if your vehicle requires this.