Look for blocked drains at the bottom inside of the l/r door. If ok, pull
back the carpet and water test the car with someone inside to track the
source.

Take both parts of the back seat out, look for a hole in the rear wheel
well, you can probably see part of the hole from inside the trunk also.

Check the rubber seals around the door frame and around the door jamb
opening in the body if youe car has the dual seal.

Do you know for a fact that the carpet may be rubber coated on the
backside like my Pontiac? This way the carpet won't get wet from water
coming in a hole in the floor. Is the floor pan rusted out?

Shep mentioned to check the drain holes in the bottom of the door, the
drain holes tend to be on the outside of the rubber seal surrounding the
door frame, but if the seal is damaged on the very bottom of the door
its possible water draining out the door could enter the cabin.
Generally very little water goes in the door from the top end thanks to
all the seals against the glass.

Does this car have those wide moldings on the lowers part of the body &
rocker panels ? These area's were prone to rot away on the the 92 & up
body styles.

Does the car have a moonroof? There are drain tubes that I think
discharge in the the rear wheel well area and perhaps they are clogged
up in some way. Check the drain tubes in the moon roof track - blow out
with compressed air and have somebody feel for air coming out in and
around the rear wheel well area.

Good Luck

harryface
05 Park Avenue 52,397
91 Bonneville 307,997