Car is 2 1/2 years old, but this has just started, turning on the heated rear screen turns off main multimedia screen. Turn off rear heater and screen comes back on! If I'm playing radio while doing this it just fades away or when I turn off the heated screen the music slowly fades back in, hard reset of multimedia screen fixes it short term but it randomly comes back,

 

I remember reading that the radio antenna is built into the heated rear window, so that may have something to do with it?

 

Here's another thread on a similar subject -

 

Sorry - just read your message again... No idea why the heated rear window would make the screen turn off.