(2008 GL550) Now Backup camera does not work, dashboard cluster will not communicate with the main radio/nav cluster, steering wheel functions (buttons) will not work and the mirrors will not automatically fold in. I have synced the sun roof and steering wheel but not sure where to go for the rest... The "dealer" has worked on for 2 days so we took back home as they just wanted to replace everything not working.
Seems all "automatic" functions are not working.

Any suggestions on a reset? or more sync dialogue?

But he wasnt having these issues before.

Was it a coincidence that the auxiliary battery died at the same time as he was having the main one replaced?

i would guess the aux battery was already dead - so when they removed the main battery, no more power to the electrical systems for an extended period of time. I also thought the aux battery was only for allowing the electrically actuated automatic transmission to shift if the main battery died, but that really doesn't matter in this context.

I understood the best practice was attach a battery tender to the GL before replacing the main battery, to make sure you don't lose any settings... That's what I did when I replaced mine...

Yes agree with prior posters, they broke it they fix it.