Hi there,
Thank you again for the tips; the door is working again

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Ultimately, I used a piece of welding wire to push the child safety lock lever into the unlocked position from the inside. However, it probably would have worked just as well to pry it open with a piece of metal from above, next to the window seal.
I found it interesting that the mounting screws for the door handle on the passenger side have a left-hand thread (which wasn't the case with the 32B, for example).
The cause was relatively easy to find: a tension sleeve that holds the unlocking lever of the door handle at the bottom had rusted away; thankfully, Lopez

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I'm starting to seriously wonder if I should switch to using Japanese rice cookers. At least they still invest a tiny fraction of a cent in the surface treatment of those parts.
Regards,
Olli.