Since this was requested, and I had been meaning to write this up, I'll try to share my notes now/here. It's late, please pardon any typos.
Most of how to pull things apart is covered in this well illustrated thread on another forum:
I ended up replacing the resistor harness, the resistor and the blower motor. The plug on the resistor harness was quite literally toasted and not re-installable on my van. I think the resistor and fan were still working correctly. However, the contacts on the resistor had seen better days, and the fan had run through quite a bit of its brushes.
One should probably not expect to be able to pull this all apart on an older van without replacing at least something. It's not that much work to do it multiple times, I did, but getting the fan in and out is a bit of a pain. I also cross threaded one of the bolts holding the accumulator in place and had to chase the threads in the welded on nut from the other side in the fender. There's not much room and that probably took more time than everything else put together.
I also discovered some long dead critter under the battery and had to clean that up.
Without making any mistakes, or encountering any project creep, it's about 10-20 minutes each to pull apart and put back together.
When this still didn't quite get the airflow and cooling I recalled having a couple of years ago, I pulled it all back apart and cleaned the evaporator.
I didn't go overboard on it, since I know most of what was in there was going to be sand and dust.
First I vacuumed it out as best as I could. My shop vac isn't super powerful, but I got most of the loose stuff out.
Then I flushed with large volumes of low pressure water. This does result in a little bit of water coming into the passenger footwell. I dealt with that by putting a couple of rags there. Doing this in the summer meant that it all dried up in a day or so. I left it all apart for a night and half a day to dry out.
I went with a high volume instead of a pressure washer since I did not want to risk damaging the evaporator. I'm not sure how serious that risk actually is, but didn't want to find out. I found some stories of folks pulling the casing apart with everything connected and was prepared to try that if I didn't get the results I wanted with this approach.
Once it seemed dry, I re-assembled everything and _finally_ had the airflow back that I wanted. Prior to this, even the top speed of the fan was much less than desirable. Another side effect is much colder air.
So, for my situation at least this was a hugely worthwhile project. If I had to do it again with another van, I'd just get all the parts to begin with and possibly pick up a high HP shop vac.
I may also stick a bit of foam into the air intake the next time I go anywhere really dusty. Since cleaning it, I've only been in the eastern Sierra and the Bay Area, which aren't really that bad.