Italian law for passport is very clear especially when you have a child who is still a minor and live with you. When you have to renew your passport and have a minor children you need to obtain the consent signature from the mother of your children and that is to protect the children to be kidnapped by one of the parents. I worked for the Consulate so many years and I have seen so many case such as kidnapping a child by an Italian or American parent. Having said that if you cannot obtain the consent from your ex wife because you don't know where she is then the Consul General of Italy has a function of Giudice Tutelare (Judge). You need to apply for the renewal of your passport and attached in writing you need to give the reasons why you cannot obtain the consent of your wife. The letter should be address to the Consul General. The Consul General take your case in his hands and can decree the renewal of your passport in the capacity of Giudice Tutelare.