How do you turn the IRT on for rear seat DVD system in Saturn Outlook?

-I have a rear DVD system. We have headphones that did work but are now not working. We have not used them in a while so we replaced the batteries and the power lights are on but no sound. When we turn on the unit in the bottom left it says IRT Off. I assume this has to do with the infrared and that is needed for the headphones to work. We bought the outlook use so we have no remote but I would think I could change what I need to using the buttons on the unit since something has changed now and we have never had a remote. Please help! We are tired of listening to children's shows and would like to have the headphones working before a long summer trip. Thanks!