Water Leaks

Have a 00 Malibu LS w/ 3.1L. When I'm parked level or uphill and it rains water leaks into the passenger compartment soaking the carpet. When it's parked downhill no leaks no matter how hard it rains.
The rain is leaking into the blower motor since when it leaks into the car and I turn on the blower I can hear water swishing around and it strains from the resistance.
Anyone have this problem and/or know how to fix it?

I would check any gromets in the firewall and replace if damaged or push back in place if needed. Also check where the heater hoses go into the firewall.

Unless you have a sunroof that doesnt drain in the a-pillar correctly(but unlikely)