Central Locking System
 

I have a 1980 450 SL. When I unlock the driver side door, it unlocks the passenger side and the trunk. Perfect! But when I lock the driver side door, it locks that door and the trunk, but not the passenger side door. Any ideas why this is?

You probably have something wrong with the "unlock" side of the diaphragm in the passenger door or the line that leads to it.

It is most likely the little spring for the emergency passenger side opening that has broken. It is located in the locking shaft between the vacuum bellows and the lock. Unfortunately , that little spring is not a replacement part. You'll have to make your own or buy the whole assembly.