[john walker's workshop]

Yellow wire gets power when key is turned all the way clockwise, not just with ignition on. If you get battery power into rhe switch at 30 terminal, you should get yellow wire power out at 50 terminal. Could just be a bad switch.
Stick a pin in the yellow wire and jump 12 volts to it and see if the starter works. The yellow wire is often cut and each end run through an alarm module. Any sign of splcing?