Trunk Leak

Hi. I was wondering if anyone has ideas. I have a 99 civic and find
after a rain there is water in the trunk. From seeing in the trunk
where the water is flowingit seems from the driver's side and I am
guessing the "pressure vent". I bought the car used and noticed this
was missing so I bought one. I am not sure how to put it in correctly
so water can't enter through there. Can anyone help? Also in case
this doesn't end up being the source, does anyone have other ideas on
what could cause leakage from that side?

 

taillight gaskets? thats a common prob on older hondas. take out the
taillights, look at the foam rubber gaskets. rub em down with some clear
or black GE 20-yr silicone caulk, then run a small bead on top.
reinstall, and tighten. the vinegar smell will go away after a day or 2.

 

Through the taillight assembly is a common route. Another is through
the drain tubes for the sun roof of you have one.

 

OK, now for the redneck way, (if you can fit)
Get into the trunk, close the lid and have someone you TRUST to spray water
on the car. Start around the taillights and work up, On my 75 Celica, it
was the rear window seal leaking.

 

could be taillite gaskets like someone mentioned but don't waste your time
reusing them or coat with silicone cause they will be most likely be
cracked
just by new ones from dealer they are only 5-6 dollars each and could
save you future headaches