I have a 1998 Buick LeSabre that leaks like a sieve when it rains. When it’s parked on an incline or flat surface. The floor behind the drivers seat will fill up with water. Do you know if this is a common problem with this model or is it just me? Do you know of a way to determine where the leak may be coming from? It’s a mystery to me.
Similar problem with a 2000 Blazer.
I used a garden hose to pour water into the windshield cowl while looking under the dash with a flashlight. I found a trickle of water coming through the seam between the inner fender and the firewall. The water was running down the firewall, under the carpet, and coming up through the carpet around the seat. There are a number of openings in the firewall to check, accel cable, steering column, etc.
Ed’s suggestion is a very good one. Sit in the back seat of the car and have someone spray it down with a hose, you might get wet.
If it has a sun roof, I would start there. A blocked drain can cause this, of course there are lots of other possibilities.