Let's run through this again.
You want the ignition switch out where you can get to the back
Turn the key to "run."
Take a light/ meter, and see if you power at RED
With the key in "run" you should have power on the big BLACK and the DARK BLUE
Now go out to the engine bay. You should have power out there at the DARK BLUE to the coil resistor
If so, so far so good. IF NOT stop right there and find out WHY
When you turned the key to "run" if the RED went "cold" then you have a problem with the RED feed. Either the ammeter circuit or the bulkhead connector is AFU or if this car has a fuse link, feeding the battery INTO the bulkhead, it may be bad
If you DO have power on the big BLACK and DARK BLUE but you do NOT have power out at the ballast resistor, then you have a BULKHEAD connector problem or other issue.
After that, check the brown and yellow. These should be hot with the key in START. If they are, get someone to hold the key to start, and go around to the 'bay. The COIL side of the ballast is where the brown goes, so see if it's got battery voltage there, key in "start." If so, so good. If you had it at the ign switch, and not at the ballast you have a BULKHEAD CONNECTOR problem, or a loose connection right at the switch
The YELLOW goes to one push-on connector of your start relay. Check this, key in "start." If you have power and the engine won't crank, either the relay is bad, or the neutral safety switch is not "made." Pull loose the "other" push-on terminal of the relay, hook your meter/ light to the exposed terminal of the relay, and with the key in start, you should have power. If so, clip lead this exposed relay terminal to ground, and the car should crank with the key. This points to the relay is OK, bad neutral safety or wiring.
If you clip lead the terminal mentioned to ground, AND you are getting power on the yellow AT THE RELAY and the engine won't crank, replace the relay.
This is easy. This is troubleshooting 101. Print out those diagrams, and go to MyMopar and download the '66 shop manual which is DIFFERENT but similar. It should give you some other ideas
http://www.mymopar.com/downloads/servicemanuals/1966_Plymouth_Service_Manual.zip