66 Cuda: horn works, no other power, no start

The thing is, you said you have no lights. This indicates a failure in that major circuit

CHECK these voltages under "failure" conditons. That is, example, if you have voltage at the ammeter, and you don't think the main splice is bad, then the lights should work

So turn them on........do they work?

If not, now RE check at the ammeter and see if you have lost power. If not, it's "further along."

"Wiggle" test the bulkhead connector.

The odd thing is the horn working. Originally, on my 67, power for the horn was pulled off the alternator

This means horn power had to go through the bulkhead.....through the ammeter.....back out the bulkhead...to the splice in the alternator output.