Yeah, I've googled this a fair bit and seems like there's a bunch of options for what could be causing the problem. As far as I can tell, the second fuse box doesn't contain any heating-related fuses, but I might as well check that anyway. There is a fuse for recirculated air / AC, but my car doesn't have AC and the recirculated air works, so that can't really be it, I guess. The actual heater fan is apparently located behind the glove compartment, along with its resistor, so I guess I'll start with taking those out and plugging a 9V battery into the fan to see if it works, and if it does, then I'll measure the resistor with a multimeter to see if that's broken. If both of those are intact, then the problem pretty much has to be in the switch, and I don't know how difficult that will be to replace. Probably not that bad. Or it could be a broken or corroded power cable somewhere, I guess.