The FM Modulator plugs in-line with the antenna lead to your HU. It generates its signal on a set FM frequency. The one at
www.partsexpress.com (part number 205-215) generates a signal at either 88.7 or 89.1 on the FM band. You tune your radio to the appropriate station to listen to whatever you're piping into the modulator on its RCA inputs (TV Audio, CD Changer, mp3 player, whatever).
You're welcome.
