The trunk of the 83 240D I just bought wouldn't open with key or vacuum. This is the second 123 trunk that I had to break into. The first was my 83 300D. This time I took photos.
Pic below shows position of the "tool", which happens to be a piece of bamboo. You can use a furring strip or a broom handle. Furring strip is actually better because it's flat. That's what I used the first time on the 300D where the cover was already off. I just shaped the furring strip so it could get a "bite", didn't cut a slot and it worked fine.
Pic below shows the hook to pop the plastic cover for the lock mechanism.
It is made from a coat hanger bent to shape and the point sharpened and bent to get under the cover to pop it off. You'll need a flashlight to see. Work after the sun goes down.
Pic below shows detail of the tool with a slot cut into it to hook onto the vacuum lock actuator rod.
Pic below shows how the tool should sit. Note the vacuum lock actuator rod (with the spring) in the mechanism has to be attached for this to work.
Standing outside the door you have to pull the tool toward you thus pushing the lock rod toward the fuel filler while an assistant pushes on the lock button to open. I didn't have anyone to help and used a bungee cord to leverage the tool.
Try it! Better than drilling the lock out.
Discuss this DIY here.