It's possible that you have a drain hole blocked which is causing water to enter the interior and by gravity run down to the lowest point.
Also it could be a fault/leak in the heater matrix or a part of the pipework.
As for the door seal, I would have thought the amount of water it would take to get to the drivers footwell would show a puddle in the passenger side.
If you're inexperienced at bodywork & mechanical work then it would be a good idea to find a competent mechanic or independant Porsche specialist.
I hope this helps.
Pete
January 2010