Who other than David offered the simple suggestion of replacing the struts that do the keepy-uppy work of the 5th door. I see cars as mobile enigmas; for him they are projects. These struts seem to be both rather affordable and easy to replace. That sounded spiffing! And thanks to him referring them as struts I now knew what they were called. It can be a nuisance to try to find information on things of which you don't know what they're called! And that is a common feature of car parts. In my world, at least. But armed with this new knowledge I went to eBay to see what I could do. And saw a pair of struts. And bought them. I thought. I hadn't paid enough attention; the picture was of a pair of struts, but the description was singular. The cheeks! But it promptly arrived.
I found a description of how to replace them on internet. I love internet in such occasions! There is hardly any chore you could ever need to do for which you can't find a guide online. And it looked simple. And when the rain abated a bit on a Sunday I arbitrarily chose the right strut and replaced it. And the door stayed open! On a damp day, on a slope! Sometimes the simple things in life can be so rewarding.
Look! No hands!
The replaced culprit
It had been a calm day, though, and it's nice if your tail also stays up in the wind. When I was packing my car for the next underground trip, I noticed that in some wind, the door still comes down. Bummer! But now I knew what to do: I just ordered a second strut, which arrived on the day of the underground trip (nice and quick!) so when it was time to go I just popped out a few minutes early and replaced the left one as well. And since then the door has been behaving. Bob's my uncle!