Sounds like bull**** to me too. I have never had to flush and electrical system in my life. The fuse that can blow is called the overvoltage protection relay fuse.
Is the factory MB dealer doing the work or a stereo shop? If it is the stereo shop, have it towed to the dealer and make the shop pay for the whole bill.
Thanks for the questions
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM