I know I'm not the only one, did some preliminary research, didn't find much. Pulled taillights out and applied what I thought would be a sufficient amount of red silicone sealant to the base of the studs where they meet the plastic of the taillights, re-installed in car, and the leak is still happening, and it definitely is not a small leak.

If you have this particular model, you'll know what I'm talking about when I say it's blatantly obvious that they came from the factory with the intention of the water running in behind the taillights. It's also real obvious that the water shouldn't be getting in around the taillight studs, but should be routed around the studs and out the bottom where the taillight meats the rear bumper.

The weatherstripping that seals the hatch looks immaculate, so I have a hard time thinking that would be my issue. After even a light rain, I can see the trail that the water is leaving, running under the spare tire from both sides of the vehicle.

In sum, I'm fairly certain it's getting in around the taillight studs but would appreciate some input if anyone has found a way to successfully seal them as my silicone job didn't do the trick.

Thanks for the time, help me save my Mazda from rusting out!

 

Get the garden hose or a watering can and apply water over the gap. See where it enters.
I don't have the 5 so I can't be more specific.

 

I recently just bought a p5 too. Nit impressed with the design of the body on these. I really should of had it inspected pre purchase. But 140,000 km for 1500 I guess it's to be expected. I had a large amount of water where the spare and jack are. Check your wheel wells. I found a number of holes in mine.