Ahh I just read it again and see he's getting water in the trunk. I read it as getting water in the taillights (which is also a common problem). There's not a lot you can do, since it has to do with the way the light housings fit (or rather, don't). You might be able to put some more weather stripping around the trunk opening and especially around the tails..
Aftermarket rarely ever fits like OEM, which results in problems like this.
Aftermarket rarely ever fits like OEM, which results in problems like this.