If your vehicle is showing trunk or spare tire well filling with water, here are the most likely causes and the parts you'll need to fix each one.
The foam or rubber gaskets behind the taillights degrade over time, allowing rainwater to run down the trunk channels directly into the spare tire well. This is also a frequent installation error if the taillight was recently removed to replace a bulb and the gasket wasn't seated properly.
If your vehicle has a sunroof, the rear drain tubes can clog with dirt and debris. Water overflows the sunroof track, travels hidden down the interior roof pillars, and pools directly in the trunk.
The main rubber seal around the perimeter of the trunk lid can tear, flatten, or lose its elasticity, allowing water to bypass the seal during heavy rain or car washes.
Located hidden behind the rear bumper, these vents allow air to escape when you close the doors. The rubber flaps or perimeter seals degrade over time, letting water from the road splash up into the trunk.
Rubber plugs at the bottom of the spare tire well can fall out or be intentionally removed to drain a previous spill and forgotten, allowing water from the road to spray up into the trunk while driving.
The urethane adhesive sealing the rear window to the body can fail, often due to a poor installation during a previous glass replacement, allowing water to leak down the interior panels into the trunk area.