Hatch seal, rear wiper seal and tail lamp seal are the three entry points to be checked and repaired. Not only do these commonly kill the TLFM but they also usually cause some of the hardest rust to repair on this car.
I wouldn't drill more than one or two weeps in the bottom of the housing... I'd worry more about keeping the water out to begin with. Place it in a zip lock bag with the mount at the bottom of the bag, then put the screw through the bag in that one spot. Leave the mouth of the bag (which is at the bottom) open and run the harness in through there. Remove any harness hangers which hold the harness too high as this gives water something to run along and follow into the TLFM.
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