Sounds to me EXACTLY the same problem I had last month...
Anyway..I was looking for something on the floor in the passenger side footwell and noticed that it was soaking wet.. So I came on here and posted a thread..
After a few people saying its my heater matrix, I decided to have a go at fixing it.. So I took the bottom half of the dash off.... and got squirted with coolant!
The heater valve (which is attached to the side of the heater matrix) had a small crack in it..GREAT I thought..
So I followed this guide the best I could and its all sorted now..
Heater Matrix Radiator: Fitting A New One - Alfa Romeo 145 - 146 Forum
I left the carpets up for about 2 weeks until the floor was properly dry..The sound deading under the front passenger footwell carpet takes ages to dry (it is basically a big sponge) so id suggest using a few big towels to soak up the water from there.
It took ages for me to get a replacement heater matrix (even though it was just the valve leaking, you cant buy the valve seperatly, you have to buy a whole new heater matrix which comes with the valve).
You can either go to the Stealers and get one for about £80-£100.. OR do as I did and find someone on ebay who is breaking a 145/146 and get it off of them. Thats what I did and it only cost me £25 and my time..
I originally went to Andrew Page and ordered a new matrix for £56 I think, but when it arrived it didnt have the valve on in so they gave me a refund, so I wouldnt suggest going there.
If your valve is leaking like mine was (i.e. had a small crack in it) the best thing to do is to keep the heater knob on hot all the time (as this keeps the valve open and water flowin through the matrix).. Otherewise there will be a build up of pressure on the back of the valve and it will squirt out like mine with the pressure behind it.
Now if you arent going to sort the problem out straight away id suggest bypassing the heater matrix..
This is done by getting a 16mm (inside diameter) hose and linking out the two pipes which come off the rear of the thermostat housing into the car - into the heater matrix (you will need to take the battery out to get to them comfortably and maybe even take the throttle body off aswel - thats what I did and it made it a hell of a lot easier to get to).
The reason for linking them out is so that your coolant flow is still in a complete circuit and your car wont overheat. If you do.. this make sure you bleed the system properly.
If you take it to a garage and get them to do it, they will charge you a fortune in labour as this job is quite time consuming, id defo reccommend doing it yourself if you dont want to fork out alot of money..
Id get it sorted soon though as this isnt a job you want to leave till winter!