If you've got an older vehicle like I do, keeping your interior dry can be a challenge. Even newer vehicles are often plagued by leaks, but luckily it's not too hard to find these leaks and fix them. I've compiled a list of some obvious, and not-so-obvious reasons your car is filling up with water, and what to do about it.
Check your rain channels!
You have rain channels in your trunk, under your hood, and if you have a sunroof, check those too. Try cleaning these channels out, because they can easily get clogged with leaves or other debris that cause leaks. The easiest way is to take a non-abrasive brush and get your rain channels clean.
Check your vapor barriers / weather stripping
Check the bottom of your doors to see if there is excessive moisture. If there is, you probably need to replace the plastic seal in your door. Unscrew the screws holding the door card in, and inspect the seal. If there is any imperfections, remove it and replace with heavy-duty poly. Check the weather stripping for the door to make sure it's not cracked or too eroded.
Check your tail lights
Depending on your vehicle, your tail lights may actually be allowing water to seep into your trunk. This is especially true for after-market tail-lights. A good way to check this is to put paper towel behind your lights, and check the paper towel after it rains to see if moisture is getting through. If it is, buy some sealant and re-do the lights.
Check your windshield
Sometimes the windshield seal can go bad due to erosion. This occurs mostly on older vehicles, but you should take it to an auto-glass specialist for a cheap and easy fix.
Make sure these problems don't go unattended, because moisture causes mold! Ask me how I know.