[strictlypersonl]

It would help if we knew what kit you have or what type of wiring you're using. Do you have a wiring diagram of the circuit?

Since the fuse pops only when you press on the brake pedal, the short must be between the brake light switch and the tail light.

If you have a test light, attach it across the mounting clips of the fuse. It will light brightly when there's a short downstream, a little less brightly with only both tail lights working. Wedge something against the pedal to keep the brakes on.

Disconnect both tail lights lights first. The short might be in one of the sockets.
If the test light is still bright, follow the wire's path forward until you get to the switch. There may be a relay between the brake switch and the rest of the circuit too.