[mockingbird812]

Is all of your "nice original wiring harness" and wire plugs in good shape? Do your wires from the back of your car terminate in a plug which plugs into the light switch? Are the lights at the back of the car grounded well? It sounds like your tail, and license lights wire are connected to an always-hot power lead at the light switch. Have you double checked your connections at your light switch? Not sure why the dash lights were always on before and not now. Do you have a wiring diagram for your 'stang? Very helpful.