If it truly is Fuse 9, as shown
here, then I'd try a new fuse before going any farther. I've had them fail and a replacement last for years.
But it could also be other things. For instance, the thermal limiter in the resistor pack is a known problem. They just get old and die, which kills the blower. Or, it could be that a resistor failed in the pack, which kills that speed but not other speeds.
And, if the new fuse also fails then there's the question of how quickly. If immediately then you obviously have a short. But a bad motor, or one that has worn bearings such that it is dragging can pull so much current that it'll blow the fuse. In that case try different speeds to see what happens.
Does that help?
Gary, AKA "Gary fellow":
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Dad's: '81 F150 Ranger XLT 4x4: Down for restomod: Full-roller "stroked 351M" w/Trick Flow heads & intake, EEC-V SEFI/E4OD/3.50 gears w/Kevlar clutches
Blue: 2015 F150 Platinum 4x4 SuperCrew wearing Blue Jeans & sporting a 3.5L EB & Max Tow
Big Blue: 1985 F250HD 4x4: 460/ZF5/3.55's, D60 w/Ox locker & 10.25 Sterling/Trutrac, Blue Top & Borgeson, & EEC-V MAF/SEFI