Water ingress into the rear footwell isn’t uncommon on the mk7 and mk7.5 Golf, which is essentially the same car as the
mk3 Leon under the skin. In the Golf, it’s usually the rear door speaker seals having failed that’s the cause of this issue. When this happens, some of the rainwater that runs down the outside of the windows and into the inside of the door cavity (this is normal) then drains into the car’s cabin past the failed speaker seal rather than exiting through the drain holes in the bottom of the door onto the ground outside the car as it should do.
I dare say the Leon uses the same type of speaker seal to the Golf (it might actually be the same part), so that might be the cause of your water ingress or
problem if the seals have failed. If so, then they’ll need replacing. If you’re getting puddles in your car, then you may also need to replace the sound deadening material under the carpet - or at least remove it from the car and dry it out thoroughly - as it’s likely to have become water saturated, and longer term might become a breeding ground for mould and bacteria.
Also worth checking the drain holes in the bottom of your doors aren’t blocked; if they are, ensure any blockages are removed.
Worth checking your rear door seals too - if any have become distorted or worn over time, they may no longer be effective and if so, the could be allowing water into your car’s interior.
