Mgc ; you've gotten excellent answers above. To check the neutral safety switch, negative battery terminal can be off, unplug the 'G' wire, likely dark green, from the starter relay. Connect a test light between the positive battery terminal and the dark green wire. It should light up when the trans is in park or neutral. It should be off when in any other gear.
Do you still have the ammeter connected and working? Wipers? Heater fan?
The yellow wire being hot in the run position has me mystified.
