If I remember correctly, you hold down the zone 3 button and then change the source to whatever it is you want to listen to: tv, dvd, am radio, etc. Can't tell you for sure because our 2506 is in for service (small things to repair after our 1st long trip after purchasing it in April), so I can't double check. I know we had this problem because my daughter put a dvd in and it was playing outside when we wanted to listen to the radio.
Hope this helped!
Rob & Pris
2016 GMC Sierra SLT 4WD
2017 Rockwood Mini Lite 2506S
30 days camped; 19 day trip to Niagara Falls