Rain water leaking into passenger compartment, 1997 Buick Lesabre
I have exactly the same problem with the same car. Previous owners garaged it and barely drove it. Now that it's parked in my driveway in the rainy Northwest, problem is a lot of rainwater pooling in the rear foot area, drivers side only.
No moisture at all on the passenger side, front or back. And none in the driver's feet area. Just the back seat, driver's side. No antifreeze smell, etc. And this is a problem when the car is parked; it's not water coming in from underneath or rainwater being driven into the car through gaps while driving.
I found two cracks in a part that I believe is called the "eavestrough." It's a single black plastic piece of molding that is screwed against the body over both doors. This seems to be a common problem -- I went to a local junkyard, found the same make, model and year of car and the plastic molding was cracked in exactly the same two places as on my car. Even with all the screws snugged, it's still a pretty loose fit, with gaps that could easily let water drip down.
After pulling off several pieces of molding and trying to figure out what's going on, here's my suspicion: Water is dripping through the eavestrough and, on the outside of the car, down the column between the two doors.
Normally this would not be a problem. But I noticed that there was a little gap between the car body and the strip of rubber molding that runs on the car body underneath both doors. When I removed this molding (it's screwed on and very easy to remove and reinstall with a 7mm nutdriver), I found that water was getting underneath the molding.
But here's the really odd thing: In addition to the screwholes for the screws holding that bottom molding on, there are three larger holes drilled into the car body underneath that molding. It was apparently made that way (the holes are painted). I'm completely mystified why Buick would leave holes in the outside of a car body like that. They're clearly not drain holes.
My best guess is that that's where the water is getting in. It runs down the pillar between the doors, pools on the rubber gasket/molding beneath the door, and then drips right into the car body through one or more of those holes. The area behind the driver's seat just happens to be the closest, lowest point in the car, so the water pools there.
Today, I sealed everything with a good, clear silicone sealer. Sealed all the gaps in the eavestrough, plugged those three exterior holes under the molding, and added a bead of sealer to the gasket below the door. For good measure, also put a squirt of silicone in every screw hole before reinstalling. I wet-vacced the carpet, which I'm trying to dry as quickly as possible.
If this doesn't work, I'll post more about next steps.
