The tail lights in my wagon used to fill up with water. I once dumped over a cup of water out of one of them. Check the seal around the lens and also look for cracks in the lens. I bought some closed cell phone once to cut my own new gaskets. New ones are spendy$$. You might also check the plugs that behind the lenses. Make sure you're wires are all good and clean, and have tight connections.

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