John,
I still might not understand it. I made a little sketch which helped. This must be an old rig with a basic turn signal device in the cab. I don't think you can achieve what you want without changing the turn signal switch.
The setup you have might be a four wire turn signal unit. This unit is not sophisticated enough to combine stop with turn which is what I think you want to do.
You have all the hardware on the back of the truck to rig it up in a conventional manner...all that is missing is a newer turn signal unit. Unless you want to keep some originality with your existing unit, changing to the more modern one would be the best move.
If I drew the sketch properly as I interpreted your layout, I would think this sort of dangerous lighting arrangement.
Do I understand all I think I do!!
Stuart
