No VGA signal from MacBook Air with Ubuntu 12.10

Question

I have a Macbook Air 2012 running osx and Ubuntu 12.10 side by side. The only problem on with Ubuntu is, that I can't drive an external monitor or projector via VGA. Under OSX this works so there is no hardware defect. Moreover, I can use the DVI dongle to connect to an external monitor. I tried various carndr --output commands. The problem seems to be that xrandr -q does list the VGA as not connected. I'm running both the 3.5 kernel from the distribution as well as 3.7.1. Moreover I inserted the modlines into the xorg configuration. But all without success. Any ideas what to do? Reinhard