rpm750

Dumb question but I washed my truck for the first time yesterday and wanted to listen to the radio while washing. I held the PTS button with out pressing the brake to access Accessory mode. The Radio stayed on when the door was opened but it shut down about 20 minutes in. Could have been longer than 20 min just guessing. Is this the correct way to use the accessory mode? Did I activate accessory mode? Or is this a factory set up to shut down after a set time? If it is a set time can that be changed? The manual is of no help.

Thanks
Bob