I'll have to look into the wireing diagrams first to be sure, but most
GMs had two fuses. One in the fuse box for all speed except high and a
seperate fues in the engine compartment for high only. Seems odd that
but fail to work. As these cars get older the fusebox connectors get
oxidized, so try cleaning the fuse terminals. I'll take a look at the
diagrams tonight.
Did you check for voltage at the motor too or just at the speed
switch?