I have a 2003 Mercedes-Benz V220 CDi. It has developed some kind of electrical fault that two Mercedes garages have failed to solve. Basically the battery discharges if left unused for a period of time (I often don't use it for a month or two, preferring my Yaris for round town driving). Currently even using a starter pack, I can get no more than the solenoid clicking. I have had the battery replaced and the alternator checked. I leave it unlocked in case it’s something to do with the central locking. It’s getting to the point where I may have to get rid of a car that has been a very serviceable vehicle for our six-person family from new. Any ideas?