I am the owner of a 2007 Mazda 3. Ever since I bought the car, I have had troubles with my left tail-light filling up with water. We tried sealing it off where we believed the water was entering, and we even tried getting another light of the exact same model, nothing has worked. The right tail-light has no problems. I have put the two side-by-side to compare, and from what I could tell there is no difference. How could water still be filling up within the taillight?

 

Have you replaced the gasket?

Also, a easy fix it to drill some 1 or 2 mm holes along the lower edge of the tail light ( out of sight ) to allow the water to drain out.

 

The tail-light has been fixed, I do not know if the fix may be permanent but it is not leaking as of now. Thank you!
I tried my own fix with silicon and it worked really well.