Ok...this is a description on how to troubleshoot door locks. I do NOT know if the system is the same in your car...but you should probably figure that out after reading this.
Ok, first. At the vacum pump under your rear seat you'll find the first rubber connector/split with one hose in and two out. One goes to the trunk and the other one is all the doors.
Under your floor carpet on passenger side/front you'll find the next rubber split with one in, three out. One is passenger front door, one rear door and the third one goes over to the other side of the car. It's not nessesary to know which one goes where at this point.
Driver side, under drivers carpet. Another rubber split with one in, two out. One for the front, one for the rear.
You can check the system by simply starting from the pump, at the first rubber piece. Take off the rubber, plug the hole by holding your thumb over it. Try to lock, unlock the system and see if the pumps cuts out after working up pressure or buidling vacum. If the pump is ok, it should stop as soon as there is enough pressure/vacum. this shouldn't take more than a second or two.
Then by going down the line, put on the rubber piece again and remove one hose at a time you can see which is trunk/doors. Every time you pull off the hose connected with the leak your system should work as normal. 2-3 seconds and pump cut out.
After that go to the passenger front door. At the rubber piece with one in/three out start with removing one of the hoses on the side with three out. Take one at a time and you'll find out which goes where. If your system starts working normally you'll find the leak down the hose that is removed.
When you gone thru poassenger side, move to driver side. Remove one hose at a time...
This will only work if there is one leak in the system. If you suspect you have got several just do one door at a time. This is the way I did when I found one of my hoses had slipped off. It was the driver side rear door so I had to go all the way...but I shouldn't take you that long. 5-10 minutes if you got someone helping you with locking/unlocking drivers door every time you got a hose off.
One thing that you'll have to be careful with is the vacum pump. Don't overheat it or anything...They are not cheap!I realize your pump will ahve a lot to do before you find the leak
By now you'll probably got the idea about closing out the leak. When you do find the hose that got the leak...follow the hose and make sure that it's ok. If you can't find a leak on the hose or connectors (the rubber in connectors can dry and crack) you'll probably got a faulty membran. These shouldn't be that expensive to replace.
It's over a year since I did go thru my car so the positions for the rubber connectors can be different but you'll find them by following the yellow hoses.
I do not know if this is the way a trained mech would do but...in lack of knowledge I did this and my system works fine.
If you got any questions...keep posting.
'87 MBenz 190E 2,3 8vlv