If you have a local electrical shop that can bench test, that's great.
Do you have multimeter?
If not, pick up a cheap one. I like analog for cheap ones, but whatever you can get.
You can use it to check for which wires get voltage when the battery is connected and key is run, access etc.
Also with the battery disconnected check for continuity. Seeing that solenoid relay with what looks to be the start ignition wire on it, and the dash lights flickering on with the key switch accessory, I'd check the key switch to see what terminals get conencted in each position.
Get the Plymouth Service manual for '66. I prefer print but digital is sometimes useful too. Bishko is one place to buy.
You can download a digital copy from here
MyMopar - Mopar Forums & Information - Service Manuals
At the end of the electrical section are full wiring diagrams including the wire positions for each cavity in the bulkhead connector.
