A good way to check for battery drain is to remove a battery cable, set it near the post, and complete the circuit between the cable and post with a 12v light, LED, or multimeter. If the light is lighting or you're drawing more than 30 milliamps, you have a drain on the battery and can then proceed to remove the fuses one by one. When the light goes out, the circuit that is serviced by that fuse is draining your battery and you can concentrate your search there.

It could be something like a glove box light or trunk light that stays on.

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95 E320 Cabriolet, 159K