This problem seems to happen frequently on the 129's. So, try doing a forum search. But, in my particular case ( 91 SL300 ), the lamp in the trunk would sometimes stay on ( bad switch ). Of course, you could never see it. Some other issues are the aux water pump / mono valve running too long after shut down, vacuum locking system causing the pump in the trunk to run, and alarm system. We are assuming of course that you have a good battery and charging system and that your cables are connected well and in good condition. If you have an amp meter, you can try measuring how much current is drawn with the key off. It should be well under 1/2 amp.

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