I have the same exact problem now with a 2000 Regal. In fact, today I tried to locate the source of the leak using the garden hose. No luck. Although I tested the sunroof drains after blowing compressed air through them.
If you open the sunroof a trough will be observed towards the front and there will be an opening in each corner, these are the drains.
I opened the sunroof and carefully poured a pitcher of water into the trough to see if it drained into the floor board. It remained dry.
There is a seam on the floor pan and a rubber grommet. I sealed each with roofing cement although I am not sure they were leaking. Tomorrow it will rain and so I will test again. At present I have the carpet pulled back and held up with a bungee in an attempt to see the water and dry as it comes in.
Let me know if you find the leak, I will do the same!