I purchased my Odyssey in May, and it came to me with less than 20 miles on it. It is the rainy season here in Washington, so we have had a lot of rain over the past few months. Because I have the trunk liner in the back of my car, I didn't notice the problem right away, but looking back, I think I may have had water in the back of my car for over a month. When I came back from a vacation in mid December, and saw some water on top of the trunk liner. It wasn't a lot of water, and I soaked it up with a single napkin I had in the glove box; I had assumed I forgot to remove my daughter's water cup from the stroller and it spilled or something like that.

Fast forward to yesterday (Jan 22nd), and I found even more water. This time, when I started to wipe the water up, I noticed the floor mat seemed to move around a little. I lifted it up to find that the floor mat was floating on about an inch of standing water. The water on top of the floor mat had actually soaked through the plastic mat. I got out the carpet cleaner and proceeded to vacuum what must have been at least a gallon of water.

Searching YouTube, I have found that a number of people have reported this issue with 2003 - 2006 Odyssey vans in the past, and it may be possible that simply putting some joint sealer under the cargo rails on the roof may solve the problem.

I will take it into the dealership to see if the extended warranty will cover it, and I will pray that I don't have any mold or anything in the back.

Update from Mar 19, 2019: I took the car into the dealership, and they brought in a guy who put a fesh coating of sealant on a seam under the roof rails. They removed the interior panels on the ceiling, wall, and floor around the leaking seam and ran a blower to air out the interior parts of the frame, where water was flowing from the roof to the bottom of the trunk. During that process they inspected for mold in the frame and all interior panels. While no mold was found, they replaced the carpet in the back trunk area because it smelled a little funny after being stuck under the trunk liner with rain water. The work took about 3 days because they needed to order a part and we had a snow storm that prevented deliveries. The car smelled like new when I got it back. Everything was covered under the standard manufacturer's warranty because I purchased the car new and I have never gotten into an accident. In hind sight, they said a rental car would have been covered, but my wife and I have a smaller car for commuting to work, Also, the dealership was unable to guarantee the car would be covered for a rental until after it was inspected.

It has been 1 month since the repair, and I have not had any problems.

Water leaking into the trunk of a brand new Honda! Body seams which weren't completely sealed and required sealant and touch up paint by the local dealer? I was shocked that our Honda was collecting water in the trunk. The van was in for recall work (unrelated) and the technician noticed the water first when I was dropping off the car this month. Well it turns out the D-pillar body seams on both sides of the tailgate were allowing water into the trunk area. The dealer kept the car for an extra day and added body sealer and then touch up paint to the area. We had always noticed the van had foggy windows when it was wet out. What is my recourse? I've read online that others with 2018 odyssey's with the identical problem. BTW we bought the van brand new and it's never been involved in an accident.