About the Mugen RR rear 4th brake light.

[Baygon]

So how can i fix that up to unflash....?

If i remember correctly:
black wire: ground light
brown wire: running light
red wire: brake light

The red wire is what makes the flashing. Depending on what you want, swapping the red with the brown wire will only make it flash when you turn your headlights on. That also depends on how you set up the wiring with your tail lights.