After it rains or i wash my car my rear left light seems to get water in it. It has been doin that for a while and now that light is a really dark red. How can i turn it back to bright red like the other... anyone know?

 

If it's getting INSIDE the tail light, you can break the seal, polish it and seal it back up - however I doubt it'll be an easy process and it'll probably take moisture somehow...

You should just save yourself the hassle and buy a new set of tail lights...

 

if you do get new tail light they say that you should take some clear silicone and seal the area were it is glue just incase it did not get sealed all the way