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heater core

could also be the a/c evaporator core not draining properly... if its water = a/c, coolant = heater core

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The sunroof drain maybe clogged up, I believe there is four hoses, or one on each corner to drain properly. Check that.. Blow some compressed air down each of them and see if you see any water that way, if you have the carpet still out of it.

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drop the headliner, i had a similar issue. after you drop the headliner youll see the four sunroof drains, just clean um out.

What if you have the same problem but don't have a sunroof?

sometimes the cowl gets gets clogges, the area under the windshield on the out side by the wipers, there is drains under there that can get clogged.

So clean out around the wipers on the passenger side and it will be fixed? No need to drop the headliner?

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I cant believe I actually posted something right on here. :-) I am glad you possibly found the problem.

is it raining outside when it happens?

the little rubber plugs on the floor board might be missing.
that made little pools of water in my car before.

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Glad we could help you out and you found the problem.