Hello,
On these cars the alarm goes off without the siren when the siren battery goes out. Mercedes decided they need a separate battery for their alarm horn and when it goes out the lights flash, it means it needs to be replaced. In most Mercedes the alarm horn is located in the passenger's side front wheel well behind the fender liner.
You will need to remove the wheel.
https://www.2carpros.com/articles/wheel-removal-and-re-installation
Below is a diagram. I included a picture of the unit apart showing the bad batteries.
Please let us know what happens.
Cheers, Ken
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Tuesday, November 7th, 2017 AT 3:20 PM