I've had to replace both horns on my 1999 ML430 over the years. They are in a location that's pretty low versus being higher up and protected from the elements. I suppose more protection means lower volume.

You can replace them with just about any 12V horn from any car (just pick something that's worthy of your MB). (Right now I have a pair of Landcruiser horns I picked up on ebay). Just be aware there is a high tone and a low tone frequency and these are a pair to make the sound somewhat pleasant.

If you mismatch, you get some interesting sounds that get people's attention.

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Jim