So I got a cheapo undertail which has a good finish but the LED brightness leaves much to be desired.
Bought some 10mm LEDs, very bright, and installed them. However I cannot figure out how can I make the LEDs shine brighter when the brake is on.
Any ideas guys?

 

I checked the 3 wires which go to the brake light with a multimeter.

one is the positive
the other is a negative continous 12V
the other (braking) has a continuous 7v which goes up to 12V when the brake is pressed.

 

Going just by what you have posted then the easiest way would be to just use the bake light wire and ground.

This way when the LED's see the 7v they will be dim, when the brakes are applied, you get a 12v that will be brighter.

Again, this is just going by what you have posted. I havent put a meter to any of the wires on mine because my LED Tail connected right up without wiring.