I had loads of SOS errors when I was playing around with this today. The SOS errors on my car meant the Bluetooth was off the MOST bus. . .
I ended up shining light down the MOST connectors for the CD changer and the Bluetooth so I could work out what was going on. . .from this I was able to see the daisy chain layout of the system, the "loop" if you like and how all the modules fit into it.
The order of a module in the BUS does not make any difference as I can see it. You could put the CD changer after the Bluetooth or whatever, provided it was interfaced into the ring in such a way that you have light going all the way round and coming out the final stage back to the CCC.
The problem I had was that I had effectively broken the MOST bus ring because I did not have my Dension powered on. . . . . . .
It took me a while but after working out what was what and plugging the fibers together without using the MOST block (BMW have been very considerate for this) so you can plug any of the fibers from the bottom into ones at the top, or any "input" into any "output". They are effectively male and female. I eventually worked it out and then just put them all back into the connector block in the order I worked out with them plugged together outside the connector.
I sat in the back seat, mucking around and cycling the ignition on and off. You could see as soon as something was wrong as you got the SOS and if you went into the comms on IDrive it said please install a mobile phone (by this they mean Bluetooth module I believe) and there were no options available.
I played about with the ignition on and off and didn't have any issues. I think the system is pretty intelligent, so I dont think you can bust anything. If you muck the ring up, it will retry comms at random intervals. I however found it easier to simply cycle the start button to shut it all off then start it up again with a different configuration.
I think the biggest PITA danger with playing about with this is if you are heavy handed and damaged a fiber. Obviously, they look to be much more hardy than the networking ones I am used to, but if you bent one too much, you could snap the glass inside and then you are knackered. Particularly nasty if you damaged one of the daddy ones going to or from the CCC.
So as to why you are getting SOS errors??? Perhaps the CD changer is not getting power? Is it defo the right one for the car because I would expect it to return the light back and you not get theses errors. . . . .