Please delete message I will post in diesel discussion

Hi Guys
Car is a 2004 S320CDi (in the UK). We have been through 3 batteries in 12 months. Car is used regularly. Local official MB tech at dealer can find nothing wrong. Alternator has been checked. Last time the tech came out he put a meter on the battery while car was off and said it was normal.
I have read posts about COMMAND or navi unit not powering down, or faulty seat module.
Does anyone have any ideas how to diagnose correctly? This is for my Dad (I am in Canada). I would like to call an independent auto electrician and give him/her some ideas of what to look for.
Thanks in advance
Ollie

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Oliver