battery dies about every three weeks

You will need to provide more information than that. What year? How many miles? Parked indoors or outdoors?

First thing first, have AutoZone or whomever check the batteries VOLTAGE. That will tell them if the battery is just dead or not. Since its been dead before numerous times, chances r the battery is completely toast.

Your ML430 wont run without a working alternator, which charges the battery & other things, so the Alternator is likely just fine.

I'd bet my $5 that its the battery. Quick fix.

l would get it load tested, the battery that is. If the battery passes the load test, it is a draw somewhere on the electrical system. Your battery must be fully charged to do a reliable load test. Also, a voltage readout on the charging system should be done. If voltage appears to be a bit lower than normal, it could be a diode blown in the alternator. Diodes that r blown will cause a draw. If battery passes load test then there is another problem with the electrical system. A draw, or the alternator. Auto Zone or other major auto parts chains can do some of these tests, but not all. If u can find an Alternator shop close by, they can be more detailed. Auto Parts usually do not charge for these tests, & some Alternator shops do not. When l had mine, l never charged for basic diagnostics since it only took minutes to do. Good luck

You have a draw in the electrical system,,,,,,,could be a bad seat control module, that is a common problem in the C-class.
You should bite the bullet and take it to the dealer, it could end up being less expensive than throwing parts at it and still having the same problem.

asteroid is right..might be a bad seat control module.