Here's something else I'm confused about: there's only 3 wires going to the rear tail lights. One for backup (small white at bottom), one for the bright red, and one for the dim red. So I'm assuming the dim red is on when the running lights are on. And both brights go on when the brakes are on. And the right/left brights are used for the blinkers. So what happens when you apply the brakes and either blinker at the same time?

 

If you push the brake, both brake lights will illuminate. Then if you deploy a turn signal the brake light on that side will flash off and back on. Pretty sure you're right about dim being the running/tail lights, and the brighter filament being for the brakes and turn signals.

There should also be a ground wire there, which gets screwed to the inside of the tail lamp cavity.

 

I find fhis interesting as i believe in Australia ours do.
whether this is age depending or other as mentioned im in Australia
but now im curious if they also come on with parkers.
there's a way to do it as per like on 59 fairlane