I've long suspected there was a leak somewhere, but haven't located it with our dry summer. But now that the rainly season is here, I've found where it's leaking in. But short of taking apart (looks like I have to remove the blower and whatever plastic is around it) I haven't a clue. I suspect a leak around the window seal but I want to be sure before I go through that expense. Is it more possible for access by removing the plastic piece that runs along the top of the firewall? Here's a picture of the leak (and it's not even raining right now).

 

Edrach,

I've not had a look at the firewall of a disassembled Imprezza, but it almost looks like you have a leak in the cowl area. If you were to open the hood and remove all of the plastic grills and covers (you might have to take off the wiper arms first) you might find that some sealant is missing in the seam areas where the water is supposed to drain. Baring that there may be a gasket where the heater assembly draws its air in through the firewall/cowl that isn't doing its job or its fasteners have vibrated loose letting water in past the gasket.