Firstly, I don't know a lot about MB's but I do know trouble shooting methods. it sounds like an intermittant problem, not happening consistantly. (it was draining while your wife drove it but then suddenly started working ok) Can you check the current draw while the car is running, like an ammeter? Using the kill switch should elliminate everything except the batterys internal resistance. it can drain itself if it goes haywire. have you checked with a good electrical mechanic about the Saab alternator and how it matches to the MB system? there may be an over voltage or over current spike that frys the battery or another component. this would happen again even if you changed the battery, possibly.
something to think about anyway. Hope this helps.

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87 300SDL 245K (so far)
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2000 Chevy Silverado (gasser)
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Is evolution true, or is Jesus the creator. (John 1:1-3)
I would rather trust Jesus on this one,
Bill
www.BillBaileyMinistries.org