Wiring Diagram Codes
If you look in some of the service manuals, there is a chart "somewhere" before you get to the diagrams section that shows the J1, J2, etc codes. Maybe 73? Download one from MyMopar for free. So yeh, J1, etc is the circuit code. The added letter, like J2A is a "sub" circuit of the same general circuit, and 16,etc wire gauge, color, last