Ok here i'll try my best to describe the problem.

When headlight is OFF: Passenger side tail light doesn't work while braking

WHen headlight is ON: All (4) taillights are ON, But still the same one won't turn brighter when braking.

So, i I figure it's not the light bulb, what could it be. thanks a lot

 

a short in your harness maybe

i could be wrong though

 

i kindda checked all the harness related to lights, they looked fine, btw, don't even know how the hell do i remove the third screw to take off the taillight assemb.

 

wish i could tell you dude cause ive never messed with any of that on my car

 

Bulbs in your tail light are "double contact". In other words, when your lights are on, one filament in the bulb is working. When you brake, the other filament works as well, so it seems brighter.

Change your bulb, and you should be good to go.

 

Exactly ... 2 filaments in the same bulb. The one used for the brake lights is out. Easy fix.

 

does it matter, just take it to the dealership to fix. end of story

 

I hope you have warranty...

 

+1 Easy fix.

 

I think its a ground, My headlights did the exsact same thing when i forgot to put a ground on them. Or bulb, plug it into the other side and see if it works if it does work its a ground lose. if it doesnt then its the bulb

 

just change the bulb thats probably it. Switch it with the other side first though to be sure