Make sure windshield is not leaking or firewall where engine compartment to cab is. Water can only be coming from below if there's no plug. Firewall and runs down or windshield and runs down to floor
Could also be leaking from heater core (making it coolant on the floor), or heater box condensation drain is plugged (which would just be water). If you're getting wetness on the floor and car hasn't been out in the rain, it would likely be one of those two options.
Heater core and condensation plug is both on passengers side
Coolant you can smell. Different color
Looking for a Used Lucerne in your area?
CarGurus has 69 nationwide Lucerne listings starting at $2,995.
Search Buick Lucerne Questions
Buick Lucerne Experts
Related Models For Sale