"...car is parked on a incline..."
I am thinking, facing up the hill?
If this is the case, and the water is under the carpet, it is very likely that the water is running under the carpet to the back of the car. Door seals are more likely to pond on top of the carpets so have a look under the bonnet where the bulkhead is. This may be covered by a plastic cover, but you are looking to see if the drain holes are chocked with leaves. If this is clear the two cars I have done recently were: heater unit not sealed correctly, water in passenger side only.
Drivers side on a different car had three problems, Fuse box lid had a twisted seal, Cable for bonnet release has a rubber grommet round it that had been pushed into the car, and finally the bottom bolt for the wiper motor was not fully secured and letting a small amount of water in.
Lifted the carpet, removed the under felt and replaced it then put it back together. Job done.