no tail lights 72 valiant

First, do you have a service manual? If not, download one from MyMopar.

Next, learn to manipulate the bulkhead connector. These are actually (usually 3) connectors in one. You can identify the line coming out of the connector in the engine bay---which supplies parking lights, and remove that one wire. Learn to deal with these. The connector pins have a "latch" that you can release, and slide it right out.

Why? What does this do?

Because you can REMOVE the engine bay side of the parking light/ marker light wire, plug the rest of it all back together and test. This will isolate whether the problem is in the passenger cab or in the engine bay. You have already isolated the rear harness.

Has the wiring been chopped up? any added lights, accessories "in there?" tach? gauges? etc?