Water in floor boards on 2000 BMW Z3

I have been having standing water in the floor board passenger side of my car. I do not know where the water is coming from. The back of the seat is not wet but the water is standing behind and under the seat. I think there must be a drain clogged somewhere but do not know where to look. It also goes into the trunk. Please help.

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2 answers
there are some round rubber seals under my car beneath my seats,i had standing water also, i noticed water dripping from them one day as i approached my car, when i pulled the plug out there was about a half a gallon of water that came rushing out, no wonder it wouldnt dry up on its own,,any questions call jerry 901 487 4437 u.s.a its a z3
You may be right the drain ports have to be checked that may solve your problem. If they are clogged it may cost about $35 I hope this helps.