My son's 2007 C Max : I just located this problem after 6 hours work today. Searched all over the net and only came up with wrong series C Max or Focus with pollen filter housing leaking ( if under bonnet) , blocked drains ( mine clear) , heater matrix pipes ( mine good) , rear washer pipe leaking in side trim ( not mine).

Took four hours to completely take out dashboard and passenger side carpet down to metal. Used a hose pipe streaming water down the front window and had paper towel on inside at metal surface that forms the bulkhead under the entire windscreen bottom edge. After about 30 mins beading then streams of water appeared from the metal seam , in my case only up to half way but could easily affect the drivers side with car motion . 

Hope this helps others .

Will try and post some re assembly pics as two dash screw were hidden and I needed to cut a 5mm x 5mm strip of dash to get a torx driver in. Will need a trim screw or silicone putting in to hide it .

Plan to paint this surface inside and outside with gutter sealer paint like this from Toolstation https://www.toolstation.com/evercryl-roof-repair/p80073 

Evercryl Roof Repair Grey 1kg One Coat

By Everbuild Product code: 80073

Hi it appears that I have the same problem and I may need to remove my dash board as well, can you tell me did you remove the steering wheel aswell to get the dash out or will it go over the top.

Best regards Steve Neal