It depends where they put the lockout on your system. Most of the time, they install a switch on the parking brake, so it must be engaged to function. if this is the case with your system, I'd start there, in the center console near the hand brake lever.
if it's in an auto car, sometimes it will only work when the tranny is in park, so check the gear selector as well.
Understandable why you want to do this, after all, you may want it so your passengers can be entertained, just be sure you keep your eyes on the road.