Gary,
The task itself is pretty easy...explaining it clearly is the challenge. I looked up a diagram for a '66, but all I could find was a El Camino. All you need to do is install your new light and make sure the housing is grounded, or, if it has one, the green ground wire is securely attached to a clean ground.
The power wire for the light connects to the TAILLIGHT circuit. This can be done easliy with a tap connector. Those are the little blue deals that your put over an existing wire, insert your new wire, and mash with a pair of pliers. Little internal blades pierce the insulation and provide a power path to your new license plate wire.
The diagram for the El Camino shows the TAIL circuit as being a BRN (brown) wire. Light GRN is suppose to be 'backup' while BLK/YEL is left stop/turn and GRN is right stop/turn.
These colors might be wrong. The chore is to find the single colored wire that comes from the front of the truck, goes to one of the TAIL light, then across the rear to the other TAIL light.
That wire is the one you will splice into with the tap connector so when you turn the TAIL lights ON, the license plate light will illuminate too. If you have a small volt probe, you can turn on the taillights and probe each wire until you find the one that is energized.
Some members of the forum frown on poking holes in the insulation with a sharp volt probe. It is truly harmless, but if you feel nervous, wipe a bit of grease or silicone on the dinky hole when you are done.
I hope this novel helps you and answers a few of your questions.
Stuart
