Bought 2001 C320 in May of this year. Needed new cataletic converter, then a few months later, battery seemed dead. Had new battery put in. It went dead within 10 days. Back to the repair shop. He said he can't really find what is causing the battery to just up and die if the car sometimes sits for a few hours. The mechanic is suggesting some new wiring thing for the front seats to the tune of $500 each. Last week end we hit a deer and had damage to right front corner, lights shoved back into empty spot, but car ran fine, but this battery thing followed it.

She's called Matilda, the Bucket Car, and rides right nice. Just doesn't start with any degree of regularity.

 

Look under the front of the front seats. You sill see a flat black box with connectors plugged into it. Unplug the longer flat connector. This will disable your seats so make sure they are set where you want them. What is happening is the seat module is not "going all the way to sleep" when you turn the key off and it is draining your battery. Mine has been doing the same thing. I found a fix for it and will post a link as soon as I find it again. Basically the modules have constant power (ignition off) as well as switched power (key on). To "fix" it you basically cut the constant power wire to the seat and splice it in with the switched power. What this will do is elimate the drain. Once this is done it should fix it unless it is something else. Fixing mine tomorrow morning since the weather is supposed to be clear. Once the fix is in place then you plug it all back in and it should work fine (fronm what I have read). Only thing you will loose is the seat adjustment with the key off but all memory will be retained. Will post the link shortly.