The user manual of my 2006 E350 W211 only mentions the main battery in the trunk and the secondary battery in the engine compartment. But when I disconnect those two batteries (I do that every time I go away for a month or longer), the alarm kicks in. To get around the problem, I usually lock the other 3 doors with the dashboard switch, then the driver door with the compact key, before disconnecting the batteries.

So my question to the gurus here is, does the car have a third battery? If yes, where is it located?

My car has the Lojack theft recovery system, which has its own battery (even though I still haven't found out where they hid it), so I suspect the alarm system may be wired to that battery?

Only 2 batteries.

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Thx for the quick reply Doc, it's got to be connected to the Lojack battery for sure then.

Well, all the batteries are connected to the charging system, so obviously the alarm system is automatically connected to it

The thing I wasn't sure till now is that no third battery came with the car

The alarm siren on your car has it's own internal power supply. When you disconnect the vehicles' main battery, the siren thinks the wires have been cut, and automatically activates.

Ah, so there is a third battery that comes with the car after all, but that one is apparently built into the siren itself from what you say.

So the interesting question is, when that battery goes dead eventually (just like the other two that need to be replaced every so many years), does the siren have to be replaced as a unit? or you can replace just its battery?

The alarm battery is probably a NiCd or lithium battery. It should last a very long time.

-Jason

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The siren must be replaced as a unit if it fails, which they sometimes do. While not common, the majority of false alarm complaints I get are due to faulty sirens.

Thx for the reply ILUVMILS.
Typical Mercedes
In my 1996 E320 W210, one has to replace the whole blower assembly when all you need is a speed regulator! I followed instructions here and in other forums, and was able to replace just the regulator using one from the S-class W140, and the thing still works fine after a year. Crossing my fingers...