While Honda can engineer a remarkable engine with a specific output of 120 HP per litre (in the S2000), they can also fall flat on their faces when it comes to PRACTICAL things such as tail lights. The integra hatchback was notorious for leaking water into the trunk through the tail lights and it appears the RSX suffers from the same design flaw. My girlfriend, Jess, owns an RSX and was recently faced with this problem.
The issue is in the tail light gasket. The gaskets are made out of a foam material that is supposed to provide a seal between the bulb socket cut outs in the trunk and the tail lights themselves. These gaskets sucked in the integra and they suck in the RSX too. In Jess' car it got so bad that I would have to soak up the water out of her trunk after each rain storm. We recently had some beautiful spring weather, so I decided to try to do something about it.
First, I removed the spare tire, jack, subwoofer and paneling from the trunk and performed some tests. I ran water down the drainage channels on hatch and then looked inside to verify that the car was indeed leaking at the tail lights. Once that was confirmed, it was time to get down to business. In order to remove the tail lights, you have to first remove the bumper cover. This is actually pretty easy. After removing 6 screws and 2 clips the bumper pops right off. Twelve more nuts and you've got the tail lights off.
Next, I applied some silicone aquarium sealant around each one of the cut-outs in the trunk. I am hoping that the silicone will form an initial barrier and the water will be forced to flow around, not even reaching the foam gasket. Really, Honda should have designed channels to divert the majority of the water AWAY from the tail lights instead of dumping it RIGHT ON TOP of them. This is just such an idiotic design. Anyway, once I was finished applying the silicone I put the tail lights back on and finally the bumper. We'll see how it turns out!
PS - Sorry for the crappy iPhone pictures and awesome photoshop work.
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Start by removing the bumper.

Next the tail lights. You can see the cut-outs for the bulb sockets.
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This is how the water gets in.
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Here are the black foam gaskets that are pretty much worthless.
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Hopefully this silicone will do the trick!