96 1500SE headset connections

I am trying to install the Audiovox Auxillary Audio Solution for MP3 Input (FMM-100A) on my 96 1500SE. The instructions I got were to connect to the antenna leads under the seat. However, when I do that, I only get MP3 sound thru the speakers, not the headsets. Where do I connect to get it to work thru my headsets (driver & passenger)? Do I have to connect in the left side pocket or is there a connection under the seat I can use?
Thanks for any assistance.
Tom