whether with aftermarket keyless remote or by key at driver's door.

Symptoms:keyless or with key , door locks again 1 second after opening,so all (driver and passengers) has to wait at the doors to open doors quickly at once otherwise the doors will lock again,otherwise everything works.Antenna sometimes stays up(even eith radio off) after key not in ignition too.

Background:? happened after moonlighting MB tech changed my serpentine belt tensioner while there is drizzle with the driver's window open.MB tech blamed it on faulty 12 yr old aftermarket alarm system.He took the back seat out and thought the vacuum lock is on the passenger's side but I pointed out to him that Mercedesshop says it's on the driver's side.He took a look and said nothing is loose.He does not know how to fix this.

Non-MB mechanic blamed it on the door lock(may be driver's door) and not aftermarket alarm system as he says even opening with the key produced same problem ie doors closing too quickly after having just been opened.

I searched this site and someone said taking the fuse out may reset the system.

Any opinion to say whether this is a aftermarket alarm problem as suggested by the mb tec h or the door lock module problem at the front doors as suggested by non mb tech.He says he will charge me one hr labour just to have a look and takes front door panels off to check some resistance.But come back when he is less busy.

Getting a new aftermarket system will set me back 750$,approx 75 % of trade in value of the car.

Thanks.

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Tom 2010 C250 4-matic Sport (Canada)