Well I "fixed" the rear display issue. Turned out there was a remote in the center storage box and one of the buttons probably got pressed when I was putting stuff in it. This changed the screen source to AUX, thus that was being displayed. I managed to fix the settings after fiddling with the remote a bit. Seems some functions of the system are only available via this remote.
I still haven't figured out the rear speaker issue. What's strange is that I followed the speaker diagnostic procedure (hold Settings button and turn volume counter clockwise 30 clicks with engine on), and there is sound coming out of the rear speakers. But they don't work for any other audio input - radio, dvd, cd, aux
hmm in fact I did leave the sunroof open one day, but that was only for a half hour and it was a light drizzle. Hope that's not related