Stopped driving our C350 regularly after we acquired a B-Class electric. Love going 100 miles for $3. The C350 mostly sits in the garage, driven on weekends and long trips. Anyway, came home late one night, opened the garage and the C350 brake lights were flashing, intermittently, on and off. Started it up, battery was low, but worked. Drove it up and down street. Parked it. Drove it again the next day, seemed to be okay. But a few days later, late at night, I could have sworn I heard random "chirping" as if locking the doors on the car. In morning, saw rear lights trying to flash, but battery was failing. Now battery is dead.

Obviously, some kind of short somewhere. Any clues on where, what harness or relay I can start to look?

 

By any chance was the headlight switch in off position.
Random flashing of lights can be solved by leaving headlight switch in auto position.

 

You battery is probly a clue, get it tested. My switch is always from day one in the off position, dont have that problem.