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I had a similar problem on my 1991 (K5/1500 series) Blazer this morning. The fan would only run on low, and none of the switch settings seemed to be working. It worked fine last time I checked, but it doesn't get too hot here in Santa Cruz... Since we are preparing for a long trip (into the desert, etc.) I wanted to get the fan working again right away.
So, I took everything apart (including the dash bezel to get to the fan switch in the heat/ac controls) and checked for worn and loose connections on all the electrical connectors. I checked the resistor, and even though it was rusty, all of the springs were intact.
Then I cleaned all of the connections (relay/resistor/switch) and tested it again a few times. All 4 speeds working fine. I put it back together and I'm ready to roll.