vacuum door locks

If I remember right these will activate by 12v on the green wire, interupt will do the opposite. Basically you have an electrical problem or bad switch as stated above, something is telling the doors to unlock.

An option is to repace all the actuators with aftermarket, using a "central locking" on the drivers and passenger doors to control the backs as well. If you can do the install yourself it will not be that expensive. Add to this a remote keyless entry module and you can integrate it into the factory security system, or use an aftermarket security or alarm/remote start combo system.

Basically on these cars aside from the problem you have, the vacuum lines tend to crack and leak in time, and one by one the door locks get weak and then stop working altogether. If the pump goes you lose all the locks at once. I do this often in this case and this way at least you have a warranty.

Now the locks will move faster then with the vacuum which I prefer, but some people like the slower, quieter operation with the vacuum. But it could be an expensive repair, just paying someone to troubleshoot a benz is steep (well I am sure you know this anyhow).

The only other hope is someone else here is familiar with this particular problem and has an easy fix, but I doubt it. However, if someone does know ...chances are they are a member here..

Good Luck