It has been awhile since I experienced this, so I'm a little fuzzy. I would occasionally get that problem as well, which would cause my head unit to look for a CD in the dash, and if none were found to switch to radio. The fix for me has been to shut the radio off and make sure the Phatbox has no lights on. If reinserting the DMS cartridge doesn't work, I would disconnect the cable on the back side of the actual PhatBox, wait 10 seconds or so, and then reconnect it. This has fixed it every time for me. I ended up rerouting the cable in the back side, so I can more easily access it if I need to "reset" it in the future.