I've a 92 ducato and when I bought it, it was doing the same thing. Luckly it was only a matter of removing and refitting the (badly fitted)plastic heater cover in the foot well inside the van. (certainly worth checking this first) Cant help any further with details as it was over 10 years ago.
Hi.
Going to check the suggested cover and coolant as camper just back. Thanks.
If it`s not this i`ve found a fiat breaker on ebay called parts4you, they have new heater control valve for sale Poss this.
Thanks for information.
Cheers