Hi,
My son bought a 2004 c230 that has been having problems since day one. His battery died. We pulled the battery out and took it to napa to have it tested and they said it was dead and we got a new battery. We installed the new battery and it died the next day. We had the battery charged and then had it and the alternator tested and both are showing to be fine. We went through and made sure that all the lights were off, recharged the battery and it lasted about 2 days and it was dead. Each time the battery dies overnight while the car is not running. We did a drain test and found 4 fuses that are drawing while the car is off:

F57 under hood (Steering position sensor, ecm, ignition switch control module); F31 driver side(20 A ignition switch control module); F8 in trunk (7.5A Aerial signal amplifier, alarm system horn/ graident sensor); F9 in trunk (25A sunroof control module).

When we first hook up the multimeter it shows around .84 but we let it sit on there for about 15 seconds and it drops to .40. Each of the fuses drain about .08-.10.
Also not sure if it is related or not but his AC blower motor is not working but all of the AC components seem to work.

Is there any relation between these fuses?
Any help would be appreciated.

These cars use a CAN bus system and a module might be staying awake / waking up at random even when there isn't any traffic on the CAN bus.

As an example of how CAN works, this is where a module resides in a door and is fed with power / ground / 2 wire CAN communication. This module controls / power window / mirror / seat controls / door light / dome light / door lock. Messages are sent from a switch or other system over the CAN and when a module hears a specific command, it performs a function.

I'm being told that some modules can stay awake for 2 min so you might have to wait longer.

I have seen alternators have a shorted diode ( actual short not the term some apply to every electrical failure ) and cause a drain. Amp clamp the battery wire at the alternator and see if there is a drain. Also with key off, place a screw driver against the back of the alternator / pulley shaft and see if it is magnetic, is so the alternator is failing.

I have seen electrical drains come and go due to water intrusion or a failed capacitor that leaked. I'm pretty sure your type of car has a problem with water getting into the passenger side of the dash so do some research on that.

Does it have full power seats with the switches on the doors? The power seat modules on the W203 have a well known fault where they fail to shut off and drain the battery. The quick and dirty fix is the unplug the seat modules under the seat and see if the drain goes away.

A more permanent fix.
https://www.benzworld.org/forums/w203-c-class/1403233-diy-how-fix-your-power-seat.html#/topics/1403233

The blower speed controller is also known to fail ON. Easy check is to pull the cabin filter and see if blower is spinning with everything shut off.

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CENSORED due to not family friendly words

There are no power seats so it wouldn't be the seat control module. I will check the water issues and see if that could be part of the problem.

First on the list in order of most common is seat control modules and you don't have those, 2nd is teleaid control module, third is radio or command, if your draining in 2 days you have a large draw, check big consumers next , alternator could be one, instrument cluster is another , or a sam control module, or rear sam control module, if your trying to diagnose without a meter that reads accurately in ma its a lot harder to pinpoint, after pulling a fuse wait at least 10 to 40 minutes to see if draw goes a way, normal draw in Milliamps is 48 no higher than 70 no less than 40 ma , or here at the dealer we connect the sds tie into the can and can read which if any control modules are awake and online, good luck