I have had this info written up for some time and meant to post so here it is. I will link to this from our Help Page:
1963/64 Cadillac Starting - Cranking System Basics
You have a positive cable from battery to the large bolt on the starter solenoid, a yellow wire from the (R) side of starter solenoid switch to the ignition coil, and a dark blue wire from the (S) side of solenoid to the Neutral Safety Switch, which in turn is fed by the purple(violet) wire from the ignition switch. There is a large ground strap from right frame rail to transmission bell housing bolt to engine block. The frame is grounded directly by the negative battery cable which attached to the frame on the right side just in front of the radiator support under the battery tray.
The Neutral Safety Switch Prevents Cranking in “Drive” Gears
The blue and purple cranking circuit is closed when the shift selector is in Park or Neutral, and open in any drive gear in order to prevent cranking the car in gear.
Starting - Cranking
With ignition switch turned to far right cranking position, the purple wire feeds the 12 volts to the blue wire at the neutral safety switch which feeds the starter solenoid switch. The yellow wire from solenoid switch in turn provides a full 12 volts back to the ignition coil for cranking. When you release the switch and have the switch in "run" position, the voltage to the coil is reduced to 8 1/2 to 10 1/2 volts since current is then provided through the brown resister wire, which gets it's current at the pink ignition wire under the dash.
One of the most important and overlooked components in the cranking circuit is the large ground strap that runs from the right frame rail over to the large bolt through transmission bell housing to engine and starter. If this is not connected and making good connection your starter will not work. Check your ground strap and make sure it is making good connection. If all other wiring is connected correctly, you could also have a bad ignition switch or melted and shorted ignition switch connector. Remove lower dash panel and disconnect the ignition switch connector and inspect closely with a light looking for melting and shorting. If all looks good, you can bypass the ignition switch and hot wire the car as noted below.
You can disconnect the large connector on the back of the ignition switch and hot wire the car by connecting the red (battery) to pink (ignition) wire and touching to Purple (starter). I had a switch go bad on the way to a car show a few years back and made a quick 3 way jumper to hot wire the car. Here is a video showing how simple it is: https://6364cadillac.ning.com/video/hot-wired-ignition-switches
Bypassing the Neutral Safety Switch is Easy
It is just a matter of unplugging the connector with the purple and dark blue wire and jumping the connector. Be aware the car will no crank I any gear.
Great explanation of the starting system makes trouble shooting simpler understanding the color coded wiring
Nice job Big guy !