The free brown lead on the Gateway car harness can be connected to the phone mute pin of the car radio. This cable is responsible to mute the actual played music periodically when FM radio or simple CD-mode is selected (the device indicates the incoming phone calls like this). Important: Connecting this brown mute cable is not necessary and also not all car types have phone mute pin.
Please note: There is no free brown lead on BMW version (GBL3BM1/BM4)
Where to connect it: The separate brown cable (except BMW) needs to be connected to the Phone mute pin of the radio.
The position of that PIN is different in every radio, therefore you need to check the pin-out diagram of the radio head unit. If you remove the radio, you can find big white stickers on it, one of them shows the pin-out diagram. If not, then the only quick solution is to google it or to contact a service station.