The parking brake and right and left control switches are actually double switches. One half of the switch is normally closed, the other half is normally open. They each have 4 wires going into them.
I would just start by checking for power at the small terminal on the starter solenoid and working back from there. If you are getting power there when the key is turned to start, then you have a solenoid/starter problem. If no power then work backwards toward the keyswitch and battery.