Mercedes: merc 500 sel back doors locked out/central locking non-functioning, 500 sel, back doors

Question
Hi there, Robert ! Please would you be kind enough to have a look at this one for me. Recently aquired a 1993 Merc 500 sel (LWB. Awesome car - by far the best I've ever driven; kind of glides/wafts along ! Anyway, the previous owner inadvertantly set off the alarm/immobiliser a few months back and a "local" (private) chap managed to do something to the car so that he could continue using it. The problem is that the back doors are now completely locked out (the central locking is not working - have tried to synchronise as per manual, but got nothing -although lights come on/off on the driver/passenger door handles when using the key or pointing the fob at them - but nothing happens !)Obviously checked the fuses & taken out the back seat to pump the pipes coming from the "egg box" arrangement. The locks can be heard clicking back & forth, but the doors remain locked solid. Have been told it may be the actuators, but of course the door needs to open to get the panel off to get to these - or so I believe. Don't want to wreck the panels trying to get them off, just to find it's nothing to do with that anyway. Can you suggest any other options for us to try, please? I would be most grateful for any help you can give. One last thing. Apparently the rear window and front passenger seat stopped functioning at the same time as the alarm was activated (hmmm?). Having trawled your archive of answers prior to bothering you with this, I notice you advise disconnecting the battery to re-jig/boot the computer in several cases, and wondered if you think this may be an option here too. Look forward to any help you may be able to offer. Kind regards, Philip  p.s. any "ball park" on costs would also be useful - why have one mortgage when you can have two !!!

Answer
Philip,

There is a possibility that the alarm module has been disconnected. This module on my 420sel is on the passenger floor fire wall. Remove the mats. There is a plastic panel covering the fire wall. It should lift out and there are several modules there. See if the alarm has been disconnected.
On my car it is closest to the transmission hump.
Raise the hood, disconnect the battery ground then positive terminals. Clean post and cable ends. Leave the hood up. Turn off all accessories and the radio. Reconnect the module.

With the key lock each door, and the trunk.

Re install the positive then the negative battery terminals. Close the hood completely.

With the key, remembering the opposite direction than lock, unlock the door drive door and wait until all the locks cycle before removing the key.

Open the drive door only and start the vehicle. Let the vehicle idle for a few minutes. Turn of the engine and exit the drive door. Lock the drive door with the key and wait for all the door locks to cycle down.

Remove the key and using the trunk lock rotate the key to cycle the door lock open. Do not remove the key until all the doors have unlocked.

I know this sounds like over kill, but you are trying to clear the alarm.
If the rear lock buttons are still not moving up then you must have a failure of the vaccuum module that actuates the door locks.

Good Luck. Let me know what you find out.
Robert