Rain
any leak on your drivers door, or firewall. could be just about anything.
Cracked heater core
Assuming this is only happening in the summer, and it is just water, you have a plugged AC condensation drain. If the wetness smells like coolant you have a leaking heater core.
I have same problem with an 02 & 06 reg, roofrack seals are useless also the aerial can leak water in, with water coming out of interior light, silconed both items, I thinks that has cured it.
Heateer Core is gone
Pour a bucket of water into the passenger footwell
Is it just water or is there coolent in it. If it's just water than it could be condensation from your A/C. If it has coolent in it than your heater core has a leak in it
how much water does a engine hold on a citroen nemo 1-4 HDI 2009
OAT Coolant.Mix the concentration dependant on your country (lowest temperature).seehttp://citroenc3owners.com/citroen-c3-tips/citroen-c3-coolant-t40.html
If water is getting into the passenger footwell one or more of the weatherstripping has failed. Check the doors as the weatherstripping makes a funned to drain a lot of the water. If there is a leak that can be leading into the cabin of the car.
remove water pump on saxo 1.4
in the top of the black expansion bottle