I had a broken trunk lock on my 1990 190E. The key turns nothing happens, vacuum actuator does nothing from turning the key. If you lock the door you can hear the vacuum actuator in the trunk.
Ok I just got the car and I do not want to drill out the lock or inflict any other damage. I cruise the web and don't find any real answers as to how to get in and I don't have any manuals yet. I was told that the gas tank is behind the rear seat, so removing the seat would be pointless. Then I notice the First Aid kit on the back deck.
I carefully removed the tray insert I found inside, warmed it up with a shoplight to soften it up. I get the tray out and I'm in the trunk. Using a drop light I look around and find that there is a small plastic access point below the lock. Inside are 2 6mm bolts. These 2 bolts hold the body latch. Using a long and a short extension and a 6mm socket I remove the 2 bolts go outside and lift the trunk open. The trunk will open with the latch attached to the lock.
I found that the plastic guts of the lock were destroyed and that no matter what I did outside of the car would not have gotten the trunk open with the exception of drilling out the lock. I have purchased a new lock. I modified the broken lock so it is operational in the meantime. First what I did was loosen the trunk vacuum actuator and backed it way from the lock. I then made up the 1/4 inch difference that was lost when the internal plastic parts desinigrated with some rubber gasket and small hose clamp.
Now I have a trunk that I can lock and unlock manually until my part arrives.
Hope this is helpful.