I am still confused, are you saying that the way you operate the turn signals is just with a 3 position toggle switch?

power in to the middle and out on the outside posts?

if that the case then you must have 2 feeds to the middle post(brake lite feed and turn signal flasher feed)
that would be why the front light work like brake lights

I would run a 2nd bulb in the tail light for brake lite only, but you would have to run a separate wire for brakes

power to the stop light switch and then back to the brake light bulb.

and then just run the feed from the turn flasher to the toggle switch and then off to the RR,LR,RF.LF bulbs Ron....