Blower heater won't turn on

I have a 99 Izusu Trooper, my blower heater turns on intermittently, some times it won't turn on at all. I do recall it turning on when I slammed the door or hit a bump on the road. I check the fuse and that's okay. Can someone offer any suggestions?

its probably the fan speed switch on the dash. Had to replace mine for the same reason.