I'm not using my center piece from my Altezza set. But I may be able to help.
Have you tried to seal the center piece already? If you have I would take all your sealant off and try to let the water out that way. First check and see how the factory sealant looks as far as which corner the water would most likely be able to come out. Then if you can get the water out put the center piece in the sun or in front of a heater vent to dry it out.
I haven't taken any of the taillights apart, and it doesn't look like it would be a fun job, but if you can't get any water out, maybe drill a very small hole in one of the corners on the bottom so that you can get whatever water to exit. Then reseal the lights like I had done on the side fixtures. Don't forget to put sealant where the keyhole is. I'm sure that spot where the clear & black plastic are glued together is not sealed very well. Also , if you are going to submerge the center piece, don't put the whole thing underwater. There are 4 spots on the back of the fixture that have sticky black crap on it. Under that black stuff is where the red lenses snap onto the black plastic and it may not be completely sealed. I suppose you could try to reseal those spots as well, then you should be able to submerge the center piece with the chrome facing up. Then if it leaks it would be easier to determine where the water is leaking in from.
Hopefully someone else has had the same problem and can help you fix the piece. I'm just going to paint mine to match my car. HeDog
