It's strange that the cabin is unlocked but the trunk is locked. That means either the lock actuator is bad or the trunk lock is in valet mode (key slot is horizontal). If it's in valet mode with no master key, break out the tin snips.

That was a brilliant suggestion to apply pressure to the lines in the area of the central locking pump under the rear seat. Certainly the first thing to try. Remove the plastic cap over the vertical port below and in front of the gauge of a MityVac to use the MityVac for pressure.

Sixto
95 S420
87 300SDL