Hello all. I’ve just discovered this problem on my recently acquired 2015 estate. Probably about 0.5 litre of water in the carpet that I’ve now dried out , but as I haven’t found the reason it has got in, I suspect that it’ll be back. There a few comments on problems with older models but I’ve yet to find a cause on the MK5 . Has anyone experienced this, and found a solution? Thanks.

Different car but similar issue just a few weeks after picking up a new mk8 Fiesta in late 2017. Water was coming in through the bonnet release handle and flooding the passenger footwell. Dealer had to replace windscreen under warranty to solve it. Hope it's not as drastic for you. 

A/C pipes or pollen filter are first places I would check, after that the heater matrix pipes.

If the windscreen is replaced and it is not glued in properly you get water in the footwell.  My long standing water problem was solved accidentally when the cracked windscreen was replaced and the mysterious leak disappeared.  This time the screen was fitted by a competent tech.

I had a similar problem on an old VW Passat. There is a drainage well at the back of the bonnet, next to windscreen, which had become blocked over the years with dead leaves etc. Once cleared out the problem was solved. When I get my Mondeo serviced I always ask them to clean out the drainage channel, and they even blow an airline down it.