Run your microphones and music into your soundboard. Run a mono out from your soundboard to the FM transmitter with any cable that converts to 1/8 in. Tune the FM transmitter to an open station. Have the cars tune their station to the station that you have chosen.
Set up a monitor for the musicians and speakers so they can hear themselves
Make sure that you listen to the signal on the FM radio for reference. It may be clear in a. Monitor that you set up, but the real “house sound” is on the FM radio.
If the signal has static, try reducing the volume. The Transmitter loses quality if the sound that is input into it is too strong.