You have to remember that you don't have one input with a high-low-off state, what you have is two separate inputs, one for high, one for low. You're going to make your circuit more complicated adding some switching to do that. It's by no means impossible, just more complicated.
Ignore what I said before about using a single resistor. I just tested here and remembered why it's a bad idea. If your LEDs arent exactly the same, the one that needs the least current will light and the others will be dim. That is, until the bright one burns out, and then the next in line will light ... until it burns out ... etc. I made the same exact mistake a few years back, you'd think it would be easy to remember.
If you want to get creative, you could use some tri-color (RGB) LEDs. You could light just the red lead for dim, and light all three (pseudo-white) for bright. Of course this could open some cool posibilities, maybe instead of the red leads, you tie into the blue, giving you really interesting purple (through the red lens) tail lights.