Welcome aboard.
Certainly on 215 and probably on a 216 there will be a scuttle drain underneath the wiper motor area that fills with leaves, seeds, soil etc.
When the scuttle drain blocks, the water diverts through the floor pan and towards the rear of the car.
On the W210's, the water would gather in the footwells behind/under the front seats and also under the rear passenger seat.
On 210's, the PSE pump is located under the rear seat and housed in a foam boot which draws in the moisture and irons out the pump resulting in central locking and other problems. A new PSE is circa £500 from the main dealer.
Cleaning the scuttle drain was added to the 'B' service schedule (I'll guess at around 2004/05) but I doubt many cars had that item carried out.
Cleaning the drain is quite simple and hopefully there's a YouTube video available.
The drain is located below the single front wipe assembly and using the likes of a metal ruler, or the wife's favourite kitchen spatula handle is the easiest way to clear the accumulated crud by way of prodding. Anything will do as long as it doesn't break off in the drain.
To test the drain is clear, with the bonnet up, pour water on the windscreen and if you can see it flowing freely down the v-shaped plastic drain exit underneath the car in line with the drain, it is clear.
Once you are sure the drain is clear, there's no harm in hosing or pressure washing the drain to ensure any remaining bits are flushed out.
If the carpets are wet, a wet/dry vacuum cleaner can be used to remove as much moisture as possible.