He is talking about a leak into the car from like rain...not a leaking coolant system....
Is it in the hatch? There is a how to by The Man to properly fix this.....I did the cheapo method and sealed the tailight housing to with some clear silicone....Most shops do not properly re-seal a important body panel seal, which is where the water seeps in...behind the taillights...
Search for the how to, or just pull the light and reseal the panel gap the cheap way...