96 1500SE headset connections

Welcome Tom...

The Audiovox FMM-100A is an FM modulator, basically a low-power FM transmitter that you place into the AM/FM radio antenna's cable (hopefully, you've got the correct cable -- I've seen a few installed in the CB antenna's cable by mistake).

Anyway, that FMM-100A doesn't control anything related to your speaker choice. To play your FM Radio (the output from your FMM-100A) into your headsets, you will need to select your headsets as the output for the radio...

If you look at your radio there's a group of 4 push-buttons in the top-left corner of the radio's faceplate, the 2nd button in the top row should be how you select to play the radio through the speakers or headsets (the button label looks a little Greek, something like " Ω/◙< " , it's supposed to look like "Headphones / Speaker")

That ought'a get ya working.

If your FM radio plays OK through your dash speakers, and you select to play the FM radio through your headsets does that work OK for you ? (completely disregarding the operation of the modulator - tune a local FM radio station)

The antenna is not related to the radio's speaker / headset switch at all. The 'correct' antenna lead would be the one from the AM/FM antenna, this is the antenna most often mounted on the right of the bike. You'll need to kinda chase the cable rearward to assure that you've got the radio antenna cable and not the CB antenna cable...

Check the speaker and headset operation with any local radio station or even a cassette to ensure that your radio drives your headsets OK... but the choice of antenna won't play into where the radio's audio output is directed...