Problem solved: It was a bad ignition switch. I replaced the switch and everything works fine.
Seems like the root cause was moisture + this horrendous solder job.
Has anyone come across something similar before? Recently, I noticed that the A/C blower motor in my Getz was not turning off when I shut the car down. This happens around 50% of the time when I turn the car off, sometimes turning the electrics back on and off fixes the problem.
I've replaced the blower motor relay but that did not help. When I put the new relay in, it clicked and turned the blower on right away. The blower itself works fine, its different power levels are functional and I can turn if off manually.
Edit: My Getz is a 2006 model. The blower motor turns off after some time, I haven't measured but it's more than a minute or so. I've owned this car for several years, so I'm hesitant to believe this is a "feature" unless it is somehow triggered by the age/wear of the car. The ignition switch has some slack, as you would expect from a car with 220k on the odometer.
Edit 2: The problem seems to have stopped, or at lease become much more rare. Nonetheless I'll add my research below:
After scouring the internet, I've sound some semblance of an electrical diagram. It seems that the blower is entirely controlled by a rotary switch (the one in the dash), which is powered by the blower relay, which receives its on signal from the ignition switch.
What's perplexing is how ONLY the blower would stay on, and no other electrical parts. If the ignition switch was broken, the problem should have been much broader, right?