It's not difficult.
The resistor block consists of some coils of wire soldered to a board. It's supposed to be cooled by the airflow from the fan in the vent, though that means that the slow speed coils, which need the most cooling actually get the least.
You can find the coil(s) which are burned out and solder then together, or, what I did was just to replace them. I used bell wire (The stuff with the red coating on it so that it doesn't short) and it worked perfectly. I got some 18AWG and some 22 AWG to do the different sized coils. I wrapped the bell wire around the shaft of a screwdriver to make the same number of coils as the wire I was replacing, scraped off the insulation ofr about 5mm, then de-soldered the broken coil, and soldered in the replacement. Results are perfect, and I can't tell the difference from the coils that are there, except that the bell wire is a lot sturdier, and less likely to cause a problem in the future. It seems that most of the resistance is actually caused by the air gap coil rather than the length of wire, kind of makes sense for a DC fan motor.
Anyway, I think that anyone with halfways passable soldering skills could do it easily.