pecos

98 astro 2WD, 4.3 v6
I've been having starting problems on and off for awhile. The van cranks as normal but won't catch. Usually happens in damp weather. When it first started happening, I discovered that if I used a hair dryer to warm/dry the black box below the battery (ecm?), it would start and run fine. This solution has worked repeatedly for over a year now. When it won't start, use blow dryer, it works.
Recently that trick didn't work. I found a blown 20 amp fuse in the wiring just outside the fuse box (under the hood). It's on the wire coming out of the Aux B terminal. I replaced it and van started and ran fine. Happened a few more times over the course of months, and each time a replaced fuse solved it.
Today, even that's not working. As soon as I replace that 20 amp fuse, it blows. This happens without cranking the engine or anything. As soon as the fuse is inserted, it blows.
What is this fuse for, and where should I look for the obvious short? I hope once I find it, all the problems will be solved.
Thanks in advance.