led - How to ignore pulse in a circuit

I have a 12v cable that sometimes pass (75%on-25%off) and other times 100%on.

What can I put on that cable so if the input is (75%on-25%off) then the output is 0v, and if the input is 100%on the output is 12v?

Is a resistor what I need?

Here i want the purple LED only to be on when the brake light is 100%. Right now if is 75% the purple LED is flickering.