This thread helped me a lot too! On a 2007 Q7, the MMI was turning on for 4-5 secs and then switching off. The optical bus diagnostic showed that only the optical sound system (AW 47) had an error in the MOST bus. So I unplugged the MOST connector to the sound system control module (in front of the D-pillar under the side trim), replaced it with the test loop connector and the MMI booted up and all not reachable modules were working again perfectly in the auto scan.
A new CM is ~$1,900, wow ...
There was 0 water anywhere though and the 4 roof drains work well, so it's weird what caused the problem.

Anyhow, this thread was very helpful to figure this out.