Loads of good tips, couple of other possibilities to consider once you are sure it's not coolant in the footwell.
1) Drain holes in the scuttle area get clogged with dead leaves and eventually leaf mould, some years ago I took a ZX through the car wash; whilst rain had managed to trickle away, the water from the car wash was too much and the front passenger footwell flooded through the heater vent. Easy fix, clean them and check them periodically. (forget them on a 924S and water will eventually find its way onto the fuse/relay box underneath). Well worth the effort and don't forget the ones in the bottom of the doors. (forget them on an MGB and the doors rust to bits)
2) Even further back, tried everything to solve the same problem on a Fiat Uno, even changed the door seal. No joy.

Had to have the windscreen replaced some time later and that solved it, it was water ingress through the bottom corner of the windscreen seal. Worth just carefully lifting the edge of the rubber seals with thin softish plastic or even a lollipop stick (something which wont scratch - screw driver wrapped in tape if you really must) to make sure there is no rusting since they can rust through (the BX 16v suffered from this)
3) Same problem on the Mi16 with a damp boot carpet, there it was the top left hand corner of the rear window seal. Again, easy fix with clear silicone in a domestic silicone gun which forces silicone in under pressure.
Never assume the leak is near where you can see water dripping, the logical place for a leak on the back window would have been the bottom corner, it wasn't.
As suggested above, run a hosepipe over the car to confirm you've fixed the problem