...if You jumpered the Battery +, bypassing the Ign. Switch, and there is Still 'Nothing' activating...in My view, It can Only be 'A' Ground(s) is/are Not Grounded.
Get out the Volt/Ohm Meter and Check for 'Continuity' with ALL Grounds, Starting AT the Negative Battery POST and going to All Groundings!
Good-Luck with it!
MJ
...and realize, the ONLY Ground the Engine Block has, is that spindly strap at the Tail of the ZF Transaxle!! Myself, Years ago, I added a 'Beefy' Ground Cable Directly From the Frame To the Starter Motor Case. This may Not fix, in Your case, but it will surely Help the Next time You try to Start the Engine.
P.S. Grounds are Always Coming Loose on these Panteras! Check the 8-10 Ground wires going to the Stud on the Chassis UP Deep, and Under the Dash, AT the Left Corner above the Drivers' Left Knee. You'll need to lay on your back and use a Flashlight, to find them.
If I was looking for a 'Positive' Disconnect...I would Inspect the Connections on the Back of the AMP Gauge. It's common, These come loose, because of the High Heat! And second, the Junction Block at the top of the steering column, behind the Steering Wheel, wires can 'Pull-Out'.