get a changer relay mate.
this is how I did it on my 180sx kouki so that I havent had to dismiss my inner brakelights.

pinout:

30 brake light RHD right inner bulb
85 negative ground
86 ignition + this is were you want to wire in a switch.
87 constant +
87a wire that connected to the brake light

now you have a seperate controllable brake light wich wont short out once you brake or turn you lights on.

also might wanna wire in a fuse.

for a s13 you can buy a oem fog light switch to keep everything neet.