Had my 2010 C-Max 2 months and the passenger footwell carpet was soaked after a long wet weekend. Took carpet up and the insulation was saturated, couldn't find the source of the water entry after much searching in the car and online! Returned car to Ford main dealer who had it for two periods of 5 days each, much water testing, but they couldnt find the problem. No charge and a courtesy car provided so cannot fault them for that.
However, looking again today, I found the daft problem. About 6 inches in front of the passenger seat in the footwell is a small circular plastic grommet about 20mm dia. Yes you guessed it, although it looked a tight fit.....it wasnt and I reckon it was able to lift up and down in the hole under the spray from the road and through which you can see the road!!!
Resealed grommit and now a dry carpet.