I am happy and relieved to announce success! Besides the strange sidelight, which had a phase of being an indicator light today, everything else is solved.

Turned out to be the wrong kind of fuse, the one used was not strong enough for the washer motor and blew when it was used to hard. The alternate one had too much resistance for the horn. I am almost sure the underlying physics are something I learned during my studies, so.

Well, at least I learned how to read my wiring diagram and use the multimeter to locate faults!

Oh, and I solved one of my mystery switches: it was used for the seat heating. Didn't tear out the wiring so, too much risk of breaking something! The other should be from the removed cruise control.