Not knowing what year S10 you have, you can generally find the blower motor high speed relay nearby the blower motor on the firewall. The resister for the other lower speeds is also found close by the blower motor.
sounds like the blower motor switch; since it is working at the lower speeds I'm guessing, then it's not the resistor (if the resistor was gone then it wouldn't work at all). Got a question for you in return. where is the freaking blower motor at on one of those. cant find mine to save my life.
If the blower motor randomly and intermittenly quits and does not work, chances are good the blower motor itself is faulty, if the blower motor does not work on all fan speeds from low through high, it most likely needs a blower motor resistor.
If all speeds do not work, suspect the fuse, the switch, the switch connector or the blower motor. If only high speed works, suspect the blower motor speed resistor. If low speeds work and not high, suspect the blower motor relay.
If the blower works on slower speeds only I would suspect the high speed blower motor relay (under the hood in the power distribution box) or the switch itself. The blower speed resistor is working if all the slower speeds work.
If you have checked the blower motor and it does function when power is applied then I would suspect the Blower Motor Resistor Pack is defective. It is also possible the blower motor speed control is bad, but highly unlikely.