2003 E500 battery drain overnight
 

I replaced my stock Audio 50 fiber optic head unit with a full screen nav unit since the CD and Cassette did not work. Then I realized that the CD player in it was shot and wouldn't even take a CD. I removed it and installed my original unit a couple days later the car died over night. I replaced the main and auxiliary battery and again the car died over night. Thinking there was something wrong with it, I removed it and reinstalled the full screen CD only unit and I no longer had the problem. I purchased another used Audio 50 unit and that works perfectly. That next day I get in the car and again it was completely dead. This seems so odd and frustrating!

I believe I have ruled out the head unit and the switch panel as the problem. I left them unplugged and there is still a complete battery drain over a 8 hour period.

Start putting a meter on the fuse connections. There’s gonna be a huge draw or your battery is complete junk

disconnect the seat module under the driver seat and retest the amp pull when the car has shut down and gone to sleep.

Those modules are known to go bad and cause battery drain.