Mmm.
Well, as you can read above, there's no one favourite source. So let's summarise the possibles:
# missing / damaged rubber bungs, seals on the firewall
# leaking screen seal
# rust to A post (underneath the stainless trim)
# rust to firewall in box section behind steering idler / steering box
# rust to firewall in area of top link pivot
# rust to front of sill
# rust to join between sill and floor
# damaged rubber seals in heater box
# damage to heater box drain
# rust damage to front apron behind heater box / under screen seal
# damage to rain deflector on bottom screen seal above heater box.
To establish the actual cause, I suggest you remove the carpets under the aeats and in the footwells. Also remove the drop down shin bin glove boxes. Remove the insulation on the front bulkhead. Dust everything with talc powder or similar. Get busy with the hose! If you can't find anything with the hose, try driving the car in the wet in this condition. Prepare to be surprised where the leak is vs where the water appears!
You'll find it helpfull to have a copy of the spare parts manual handy as this has an exploded diagram of how everything goes together. That might give you a few inspirations!
Chris