Water in the footwell
Generally, you could find out where the water is coming from judging by the traces of water or by removing the footwell carpet and asking someone to pour buckets of water onto the car. If you have a sunroof fitted, this is the first place to check. There are water drain channels in the C pillars, if one or both of them become clogged, water accumulates above the headlining and comes down generally behind the B pillar trim.