How to fix this?

Any way to fix this without tearing the whole tail light apart/getting a new one. Technically it’s not doing any harm but it looks bad

 

Drill a small hole in the bottom so it drains.

 

Ok but wouldn’t that make it more susceptible to water getting IN?

 

There is not much you can do about that other than replace the light. Drilling a hole may let some water in, but it will also allow it to drain.