My 2003 C240 had a faint buzzing sound coming from the passenger side firewall as I approached the car, and before I attempted to start it. Then, no power for lights, gauges, doors, or start. Jumped it with a charger and it started right up, quickly removed charger, shut it off and restarted 3 times. Battery tested good, alternator 14.3v. Next day, same thing. I'm thinking it may have to do with the alarm. Has anyone experienced this?

 

Non-original alarms are famous for causing all manner of inscrutable electrical troubles. As well as hack-job installs.

If you have a non-OE alarm, and any doubts about the competency of the installer... I'd consider removing the thing. W203's in my area are averaging in the low to mid-$3000 range. Not something to buy expensive/comprehensive insurance for.

One man's opinion.