So if that connection on the relay is not connected to anything else (confirmed with testing as mentioned) then I can connect that J3 Brown wire to the J2 blue wire, and get my ignition in RUN and START with no backfeed?
You can easily test for it. If you can not prove it is not happening, do one of what I posted in the bottom of post 25.
The simple answer is this: Either your newer car has an isolated bypass contact or not. (the old IGN2) If not, then backfeed is gonna be a problem. If that is so, then you need either a diode or a relay to resolve.