1991 300D 2.5 Turbo, 125K

I had some trouble with my Central Locking system since I bought the car:
Sometimes, unlocking the passenger door, won't unlock the driver door. Also, if I lock the passenger door, sometimes the driver door wont lock automatically (I have to go around the car and lock it). Unlocking the trunk often wont unlock the driver door. I tried to find a pattern of behavior but I didn't manage to.

A few weeks ago the trunk lock stopped working with the power system. I have to manually lock/unlock the trunk.

Any ideas?

Thanks
Andreas