I was hoping you'd say you had an S-Class. It would've been simpler!

I don't know how much experience you have with cars, and there are several ways you can proceed, but here's a few simple things you can check. I'm assuming you don't have the equipment to check the actual "rest current" draw.

First, determine if the alarm is causing the battery to go dead, or, it's being triggered as a result of low voltage. The alarm siren on your car has it's own internal power supply. If it's dis-connected while "armed", it will activate. It's located behind the inner fender liner, to the rear of the right front wheel. Dis-connect it, and look for corrosion at the connector. I've seen lots of them fill up with water.

I've also seen lots of 203's with faulty seat control modules. The seats operate normally, but the modules still cause the battery to go dead overnight. It's easy to reach under the seats to dis-connect them. You can try this and see if the problem goes away.

Ideally, you want to check the actual rest current draw, and diagnose accordingly. I don't know if the guys at Advance Auto will be able to help you. A dealer or qualified indie will be your best bet. Good luck