What type of diagnostic equipment are you using? Standard OBDII equipment is of no real use with MB. Change the Aux battery and take the car to a workshop who has Star Diagnostic test equipment ask them to do a diagnostic check using their equipment. You will be surprised at the number of faults it finds.
With MB a low voltage situation affects almost all the electronics on the car and a diagnostic read out with star will show low voltage faults all over the cars systems.
Once the battery has been changed and all fault codes cleared you will be in a good position to look for a power drawdown, and a drawdown is not uncommon on Benz.
And would you really allow cheap parts on your aircraft?