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

"No lights"

This and the other problems generally denote problem in the first of the primary power

Problems at the bulkhead connector,

Loose connections at or problems in the ammeter

"In harness splice" failed. This is a factory welded splice in the black ammeter wire up under the dash." I'd save that for last. That failure is rare, but can happen.