I made a circuit using resistors, diodes and relays to turn the side markers into (running,turn,brake) lights as well as the rear left/right turn signals.

I made the center panel running/brake lights.

When the turn signal or flashers comes on, it disables the brake and running combo lights so the turn signal wins over the brake and running light. If just the running light is on when the turn signal goes on, it blinks off/bright/off/bright instead of dim/bright/dim/bright which makes it much more visible.