this sounds like the foam rubber gasket that seals the lights may have been displaced or has taken a set so can no longer form a seal
I'd suggest to remove the lights entirely and check over the gasket for rips or tears and when you refit them use a smear of black silicone sealent around all the seal edge to ensure its watertight
please note that the sealent needs at least an hour to set before testing the seal with a hosepipe
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