05 SL55, COMAND system demise...
 

It started with my COMAND system rebooting on its own,
shutting itself off, and sometimes not starting up at all.
This lasted for a couple of months, and was really annoying.
Then it died altogether, and has shown no signs of life for a while.
The local dealer said the whole system was busted, and needed to be
replaced, something I felt reluctant about, for obvious reasons.
Has anyone experienced something similar? Anyone know someone who
can help, or do you think I'll have to cough up the cash for a replacement system?

The same thing happened to my wife's ML320 when it was 6 months old. Fortunately it was under warranty because we were told the unit was $6k+.

Dealer doesn't repair. Only replace.

Someone with electronic skills and experience with COMAND might be able to help by actually fixing. But without a proper diagnose it's anyones guess.

Have you had the convenience battery in the trunk checked out?? Have you disconnected it, waited a few seconds and reconnected it to "reboot" it?

These are things you should look into when the command system starts acting up...
I hope it is something simple, but if not 2 options you could look into would be to find a used one on eBay or salvage yard and give it a try, or replace with an aftermarket stereo/amplifier/speakers....
Good luck..
Nick