I've had similar (in my 159) since the thermostat was replaced a couple of weeks ago. Getting more that a budgie's breath, more a cat's breath (without the fishy smell) of warm air, but as you say, when the fan's turned up, the cold from outside overwhelms it. It is warmer when the engine is under load though.
The temp guage sits at a rock solid 90 degrees when warmed up.
I took it back to the garage who changed the thermostat (local indie italian car specialist). They thought it was an airlock but after trying to sort it for about an hour, no joy.
It's going back to them next week for further investigation as the heater worked fine before the thermostat was replaced. The chappie at the garage suspected that the new thermostat may be faulty.
Dunno if any of that helps, but I'll try to remember to update this when it's back from the garage.