I had the same problem on my SVT. The water comes in where the rubber weather proofing from the hatch trim mates up with the hatch itself.
I replaced my lens and bulbs only to have them go to crap about 12 months later.
So I dug a little deeper and removed the hatch trim and totally resealed it with new weatherstripping and I have had zero problems ever since.
Seems to be quite a common occurance, definitely an oversight by Ford.
Tom's Tune // CFM 67 TB // VF R&DrEM // Steeda STS // Boss 302 SKnob // Eibach Pro-Kit // KYB AGX
CFM HFC & Flex // MASSIVE STB // Hella Xenon Euro-Spec HIDs // EuroStar LED Tails