An update for those searching this thread later:
I replaced the blower motor resistor with a new part. After taking the old piece out, it was so rusty that it literally crumbled in my hands. I thought the water leakage issue was for the newer generation of these cars... I suppose since the car is so old, even a minute amount of leakage and condensation can be expected and the part is ridiculously uncovered from the elements as if designed to fail.
After installing the new part, though, I still couldn't get the blower motor to actually start spinning up. So, as any do it yourselfer might do on an extremely cold day, I decided to whack it repeatedly with the nearest handy blunt object (steel garden hose sprayer nozzle) and that managed to get it spinning. So it looks like the brushes are going bad in the blower motor itself, as after I got it spinning I had all of my fan settings back.
I'm not sure if the brushes are serviceable (probably not), and I imagine that they are also the victim of whatever water is getting down there. Used parts on eBay can be shipped to me for about $40 CDN, or I can get a new part from partsource (brick and mortar) right away for $70 + tax... whatever you do, don't buy this part from Ford ($140+)