I'm not trying to be a smart aleck here but obviously you have a leak somewhere. It could be anywhere, the roof, the windows, the door seals, the floor pan, the trunk lid, the trunk floor pan, etc. , but if the moisture is restricted to behind the front seats, then I would start looking there.
Perhaps a bad door seal? Hole in floor? Rear windshield seal? Trunk seal? (check trunk area for water under spare tire etc.) Perhaps a bad door seal? Hole in floor? Rear windshield seal? Trunk seal? (check trunk area for water under spare tire etc.)
They plant their crops on the floor of the canyon because when it rains the water forms into the sand and canyons and also to collect flood water.
There are details you didn't give us.If your washroom floor drain only clogs when it rains, that may be a feature, not a bug. There are backflow traps that can be installed in floor drains to keep them from backing up and getting sewer water all over the floor. If your washroom floor is in the basement, the fact that it's clogging when it rains may be saving you from a big, nasty cleanup job.
One reason your floorboard is wet after it rains is that there may be rust holes in the floor or the floor pans are rusted out. A leak in the windshield seal may also be the cause of wet floorboards. Bad door seals or window seals can allow water to soak the floorboards when it rains.
I would check your AC evaporator drain tube , if clogged water will drip on floor.
They plant their crops on the floor of the canyon because when it rains the water forms into the sand and canyons and also to collect flood water.
Depends on where the water is? back seat? trunk? floor board? mine has a leak from the driver door seal being weak, the trunk seal the same and the rear water channels for the sun roof need to be blown out again to stop the back seat leak when parked on a hill and it rains. so to be honest you have to find out where the water is, your best guess on how it gets in (ie light rain, heavy rain, car wash etc) before people can truly help you.
There is obviously a leak in either the firewall/cowl area or the window seal.
The drain line for the cowl is stopped up or the windshield is leaking.
On my 82 280D I found the Sun roof leaked. ater resealing in. no more water.
My 1993 Bonneville can get water in the trunk from the trunk lid being opened at which time ice or snow falls in and isn't noticed until it melts.
It could be a door seal or more likely the windshield is leaking.