Mercedes Benz Other Repair Question
Sounds like it could have been a poor stereo installation.
If you live close to a walmart I do believe they are still testing batterys and alternators for free. Might call ahead to find out. Some AutoZones will as well.
You can try that approach or if you have a test light, I'd disconnect one of the battery terminals and place the test light in series with the connection. (ie.. tester to battery and cable to tester) The current should not be high enough to light the test light when it's connected this way. If it does light up, try disconnecting the wires going to the alternator, if it goes out you probably have a bad diode in the alternator. A bad diode can have a pretty good current draw, which can drain your battery down in a few hours.
If it didn't light up when you first hooked it up in series then you can hook up a mulitmeter inline (reading amps) in series (in place of the test light) to get an idea of how much current the system is drawing. It should be no more than around 75milliamps.
If it's up over an amp or so, I'd start by pulling the fuse to the stereo and see if the current draw on the meter goes down. If it does then you've found your culprit. If it don;t make a difference then you can start pulling fuses and relays one at a time (putting back the good ones as you go) while making note of the reading on the meter, when the meter current draw drops off, you can isolate which system the fault is in.
Hope this helps
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