The OEM 3rd brake light in the 2016-2018 Tacoma is poorly sealed and allows water to leak into the cabin.
The water soaks the headliner, leaks behind panels, and soaks the rear seats.
Toyota released a service bulletin in 2018 and covers the problem, but only under the standard 3 year / 36,000 mile warranty.
Water can leak into the cabin of a 2016-2018 Tacoma because Toyota did a terrible job sealing the 3rd brake light mounted above the bed. There's a gap between the LED light and the body that collects water. Meanwhile, a cheap brittle gasket does little to stop that water from leaking into the truck.
The problem only affects double cab versions of the truck, as the access cab uses a different light assembly.
Even a small amount of water can do significant damage. If an owner doesn't spot the leak right away or the water drips behind trim panels, there's concern about mildew forming or electrical wiring fizzling out.
As the leak worsens, it stains the headliner and can pool on the back seats.
The "high mount stop lamp" is mounted from inside the truck which means to service or replace it, you need to remove the b-pillars and drop the headliner. That's a heck of a lot of work just to access two 10mm screws holding the darn thing in place.
The work to replace the light is only covered under the standard 3 year / 36,000 mile warranty. In July 2018, Toyota released service bulletin #0055-18 titled Water Leak at Rear of Cab -Center High Mount Stop Lamp that outlines the steps needed to confirm and fix the problem.
The service bulletin only applies to specific VIN ranges of the 2016-2018 Tacoma double cab.
Once you're out-of-warranty the light is deceptively expensive unless you want to replace it yourself. On average the cost of repair is around $500, mostly for labor, as the part MSRPs for around $150.
The good news is while the original OEM part (#81570-04040) seems to just be held on with double-sided tape and the faintest hint of a foam gasket, the newer version from Toyota (#81570-04051) comes with a much thicker gasket.
Some say if they're going to go through the trouble of dropping the headliner they'll just replace the whole assembly with a fancier 3rd party option. While others try to DIY it with a bead of clear silicone sealant around the outside of the light, hoping that'll stop the leaks.
Generations Where This Problem Has Been Reported
This problem has popped up in the following Toyota generations.
Most years within a generation share the same parts and manufacturing process. You can also expect them to share the same problems. So while it may not be a problem in every year yet, it's worth looking out for.
3rd brake light over torqued at factory resulting in defective seal and eventually cracked light fixture. This results not only in electrical issue but serious leaking into cabin - affecting rear seat, roof, and flooring. Leak noticed after rain storms and noticing wet floor/seats and damp floors.
Rain and car wash water will come in the cabin of the truck due to a lose rubber gasket that was not fitted properly to the truck bed light. I also had Toyota check for rust in the cabin once they took the piece off.
Water came in through the back of the cab from the outside cab light I noticed it while going through a touchless drive through car wash. The water soaked through on the headliner and seat leaving a water stain and possible mold. I took the car in to a Toyota dealership and it was confirmed that the water was coming through the cab light on the outside of the truck. This problem was posted on Toyota's bulletin by other 2017 Tacoma owners.