I am having ALL the same problems!!!
I have an Iveco 59-12. Year 1991, and the tacho/speedo stopped working. As I have converted the vehicle into a motorhome, I decided to swap the tacho speedo for a non- tacho one. This non-tacho speedo jujst plugs straight in and is from a 35-10 and it was working in the donor vehicle but we cannot get it to work in my motorhome.
We have replaced the sender unit in the gearbox and still it doesn't work, we have even refitted it to the donor vehicle and it works in there too! Like you, we are getting power to the unit and have checked all the relevant fuses. My mechanic says the only thing we can do is find a wiring diagram and he says as long as its from an Iveco of a similar age it will do! So maybe we can help each other if or when we get our hands on one! Best wishes and Good Luck.
BUDESURF
February 2008