You probably have fuse spring clips which have lost their tension from overheating. Also the contact area on the clips could be corroded. A good repair is to remove the fuse panel from the housing and add an inline fuse holder with the newer blade type fuses. You could route the wires in from the top so the new fuseholder is accessable from the top. Be sure to get a heavy enough inline fuse setup.
Your heater blower motor bearings might be dry causing the motor to labor at the lower speeds which will blow the fuse.