I'm surprised that he chose to continue using your car then, because I would have thought being found out once would permanently turn him off and have him find another place. I guess you might have communicated to him that it was okay for him to use your car is a stronger way than you recall; or maybe the 80s just had a different vibe.
I don't think he had a whole lot of options in his life. Primarily I think he did it because he felt safe there, that area was at the time very much out of the way of traffic and apart from the occasional burglary there wasn't much crime.
One funny angle is that I was living there illegally myself, I didn't have a kitchen or a place to shower there and it being so quiet is why I got away with that. The people from other companies in that building knew that I was staying there overnight, but they in turn also didn't mind because they saw me as their 'free nightwatch', so there is some recurring element in there.
Anyway, it's ages ago, and if it happened today I'm not sure how I would respond, and I always wondered what happened to him but I did not see him after I moved (I went all the way to Poland).
My unit was the one one from the corner on top (the entrance was at the back), about 60 square meters, an early version of a hackerspace I guess, and the bike path on the left wasn't there at the time, that's where there was parking space.
Now there are a few thousand people living on the other side of the street so you would never get away with it today anyway, someone would spot you there before the first day was out.
This was around 1989, the location is Veemarkt in Amsterdam ('Veemarkt' is dutch for cattle market).
Back then homeless people in Amsterdam were fairly rare, now there are 100's of them.