Some basic areas to look....................

Check for voltage at the fuse.
Nothing on either side=problem is ahead of the fuse box.
Voltage on both sides=problem is after the fuse box. follow the wiring and eventually you'll find the problem.

Check for voltage at resistor block, check resistor block for open circuit. They do melt and fail.

Because the old cars usually didn't have any relays, all the power goes through the switch.....
check the switch by removing it and inspecting, they often melt internally.