The courtesy light in the rear seat will not light up when you open doors.

I had the lightbulbs tested and they are both working. The light will turn on when I push any of the three buttons but the one that says door on it. The mechanic says there is a "modem" beneath the buttons and that it might be the culprit. I have not been able to locate a site where I could purchase such "modem". Do you know the name of this part and where I can purchase it? Thank you.