You need to check to see if there is an excessive current draw when the car is shut off. This happened to me on an old BMW - turns out that the contact switch in the glove box wasn't making contact, and the light was staying on all the time. My commute back then was only 10 minutes each way, so the battery didn't fully recharge from the draw of a lowly 5 watt bulb.

Connect a current meter between the battery's negative terminal and the negative cable (disconnect the cable from the battery first). Check to see what the current draw is. Should be low - like 0.01-0.03 amps. If this is what the meter shows, then your problem lies elsewhere.

If the current is higher (mine was 0.250 amps), then you start to remove fuses to check which circuit is causing the drain. Pull a fuse, check the meter, and replace, moving on to the next fuse, until you find the culprit. The draw on the meter will drop significantly when you've located the right circuit. Then you will know what components are causing your problem.

Hope this helps,
Troy
1995 E420 89k