On my car, I have all red lenses; Parking light and directional on the outside (directional to the brighter filament), brake light only on the inside (single filament).
So, all regulatory BS aside, let's help Phil, here......
"Nothing is ever easy. OK. This is my new left rear taillight that I would like to install. Parking(head) lights and turn signals on outside with double element bulb, black and red wires. And braking light, one element and one black wire on inside.
Of course this is not just a simple switch. Current setup, all red, has parking(head) lights and brake lights on the outside with double element bulb, black and red wires. And turn signals with one element and one black wire on inside.
Not too swift on electrics. Help appreciated"
OK, so go to the lights you have. You have one red wire and two black wires on those current lights. I'm gonna guess and say that you want to just swap the two black wires on each light and you should be OK, with the end result of moving your brake light to the inside and the directionals to the outside.
But I can't be sure what color was used where on your car, so I need your help.
Have someone sit in the car and step on the brakes. Next, you'll need a voltmeter to probe both wires going to the outside of one light to figure out which color wire is the brake wire. Once you figure that out, swap that lead coming from the harness with the one going to the inside bulb (currently the directional).
Do that, and you'll end up with parking and directional on the outside and brake only on the inside, which is what I think you want.
No socket swap, just a wire swap in your case.