Get yourself a 3.5mm stereo headphone plug - you can buy them on Amazon for about $5 - and plug it into the aux input jack in the console. Then go to the Media menu and select AUX as the source. With the plug inserted, the infotainment system will think there's a real source there, but of course there's not, and you'll just get silence. Voila! No radio, and your nav will still work.