What engine is in it please?
One thought that immediately springs to my mind is that the cooling system on the Fiats - and many other makes - uses the heater circuit as a radiator by pass when the thermostat is closed. So when cold and the thermostat is closed the water pump has to pump the water somewhere so it pumps it round the heater circuit. If you block off the heater pipes then, when the thermostat is closed the coolant has nowhere to go so the water pump impellor will be cavitating and water will not be circulating - not good. I would think this is likely to facilitate localized overheating in the cylinder head and possible consequent damage to the head gasket which may well start leaking at a later date. If you've blocked off the heater hoses on the engine side of the bulkhead then this leak can't be coolant. So, are you sure it's coolant and not screen wash perhaps? Maybe it's coolant still in the heater matrix just leaking out? in which case you'd expect it to stop after a while? How did you block the heater hoses. Did you actually disconnect them? If you just restricted them in some way - like with a brake flexi hose clamp - then maybe some coolant is still getting through because there will be quite a pressure on the hoses especially when the thermostat is closed (engine cold)
Anyway, hope those thoughts are a help? Good luck.