Summary
My mother-in-law examined the circuit board and didn't find any bad solder points. I replaced the strip fuse [$2.00 at MB dealer]. Fan motor tested ok [direct to battery]. I had a starter & altenator rebuilder check it and he said it's ok.

I tried Arthur Dalton's test posted 01-19-2006, 08:16
< You check for power at the 3 prong conector behind the booster. Red/blk is 12v power and y/blk is variable power [ 2-8v] for regulator trigger. If both are good, ground blu wire at regulator to complete the switched ground circuit. If blower works, reg is bad. >
but I don't think I did the correctly.

I removed the regulator and added more dielectric grease and noticed the original grease is "white" in color. Maybe this is what produced the white puff of smoke?

It functions properly, but today is cool & dry not the hot humid weather when it failed.

So the questions remain; what caused the white puff of smoke, and the blown strip fuse?

Bob