1994 Chevy Corsica Confused

Hello, I just purchased a 94 Corsica from a mechanic who rebuilt the transmission and check out the entire car. He even gave me a 6 month warranty on the transmission. It passed inspection and was just given two new tires.

My question is,
It was parked in my driveway- down hill and the next day I went out and all the floor boards- nothing else, were soaked. The heat is hot and the car runs fine is not overheating or anything.

I have checked for leaks in trunk, but not sure what I am looking for.
I was told it could be the heater core but the heat runs and there is nothing that would make me think it was that.

Also, it does need a cv or half shaft, the previous owner said that he would fix it and that he would have fixed it but it is so minor and wouldn't be aproblem.

Also when it was faced down hill in driveway I smelled exhaust fumes in side, but when driving or facing up hill it doesn't smell. My mother said that it was because it was enclosed by the house and that her car does that sometimes.

I am just concerned as I have a preemie with lung issues and want to make sure the water is ok and how to fix it and I am sure the smell was from that but just curious
Please let me know thank you and Happy New Year.