rubber mats...

What he says!

A one piece rubber mat is the floors worst enemy. There will be some moisture on the floor from time to time due to condensation caused by temperature, weather conditions and so on. A carpet will breathe and let it out while a rubber mat will keep the floor almost constantly wet.
I have seen it with my own eyes on a '55 Plymouth I once had. This is also why the floor is always rotted on late fifties forwardlook Mopars that usually had rubber mats covering the whole floor.
A good quality rubber mat that would fit and look good would probably cost more than the cheapo carpets anyway..