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.
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