Carpet in 2009 DTS trunk is soaking wet from water leak, what's the resolution?

My 2009 DTS leaks midway in the trunk primarily on drivers side soaking the
carpet and filling the spare tire well half full. I took it to Cadillac dealer for leak
check and repair, they're leak expert said it was leaking around wheel well
sealed it and charged me over $300. With the first rain I found it was leaking as
badly as it had previously and I'm now out $300. I'm taking it back but afraid
they're "leak expert" is just guessing and will just keep taking shots in the dark
hoping one will work. The two suggestions to resolve I found online seem more
plausible to me. One is that the taillight seals are leaking, the second even
more likely is the body joint seals in the trunk area are leaking and need to be
resealed with sealant. I really want to keep the car it's in near perfect condition
other than this leak. I hate to keep throwing money at the problem hoping to
eventually find resolution, anyone have any ideas?