[Christ]

Every other row will be plenty bright enough to light up the whole lens. Especially since you can put like 12-15 rows at least of 3mm LEDs in the width of the lens.

If you don't like every other row, you can also do every other LED, staggered in each row...

So it's like
.0 (1) 0 (1) 0 (1) 0 (1) 0 (1)
(1) 0 (1) 0 (1) 0 (1) 0 (1) 0
.0 (1) 0 (1) 0 (1) 0 (1) 0 (1)
(1) 0 (1) 0 (1) 0 (1) 0 (1) 0

And the (1)'s are running/parking lights, while the 0's are brake lights.

And in the lower red section, you can make it so the (1)'s are running/parking, and 0's are turns.

Catch what I mean?

You can get rid of the hot-spots that LEDs normally have (bright point of light) by filing the tops so they're flat instead of domed.