How can I fix the driver's seat heater?

It could be the heater element or possibly an issue with the wiring. If there is power to the switch and heater element and your not getting any heat then the element is most likely the issue. I have seen both the switches go bad and the heating elements. If you have already changed the switch and the circuit has power then the heating element is what I would do next. The seat has to come out and the seat cover removed to replace the heating element. This is a fairly involved repair that requires knowledge of the SRS system (side air bags in the seat) and special tools for assembly.