The schematics of the J-Box are shown for pre-'94 and post-'93 Concoursai courtesy of Guy B. Young (no relation). If you're on the road, or in your garage trying to confirm that the J-Box is the problem, this might help diagnosis and temporary repair.
Pin 8 on the JBox connector was unused, so I scrounged in the junk box and came up with a female spade connector with a 6" piece of wire on it to install in the connector. Wrapped electrical tape onto the wire end.
First, Pin 8 provides a switched 12V output through the Headlight Fuse, for anyone who needs it. My real purpose was to circumvent JBox failures while on the road.
To fix this on the road, first pull the 10A Accessory fuse and replace it with the SPARE 30A Main Fuse, found under the JBox cover. Then run a temporary wire from the rear positive accessory lead and splice to the Pin 8 wire on the JBox, start up and ride home. Use the kill switch to stop the engine, then pull the temporary wire and turn off the ignition.
To start the engine, with the ignition on, briefly poke the short wire from JBox pin 8 into pin 13 on the JBox, and the engine should start. (Or stick a screwdriver across the solenoid terminals.)
Poke the short wire from JBox pin 8 into pin 9 and the headlight should come on. (Or ride home in the daylight.)
These steps are untested but I'd rather have some plan than no plan when my damn JBox dies on a trip someplace. At least I can use the voltmeter for something. And the little wire stub hanging down from JBox pin 8 didn't cost anything - sometimes the best things in life are free.