Jim has covered most of the issues on the front foot wells :thumb2:thumb2
Take some white kitchen roll and press into the damp carpets, check the colour and smell to eliminate coolant.
Rear foot wells,
Do you carry a dog in the back, or your granny :augie :lol:lol
If not, it could be your rear door cards, behind these there is a polythene sheet that must be sealed all around, otherwise you will get this problem.
Our Picasso has under floor compartments, which I happen to keep my tools in, and this was flooded with 2" of water. I joined the Picasso owners club and this was a common problem. I removed the door card, and I could see a small crease in the poly liner and this was letting water in.
Re sealed it, it has been dry inside ever since.:thumb2
To locate the area which is leaking, I used the kitchen roll at vulnerable points.
Also check your sills are not full of water, there are drains on the inside of the sills, with a very thin slot in them.
Some cars suffer from rotten sills and this is probably the cause.
Best regards, Rustic