hello there old buddy
2008 punto, is that evo, grande?
if interior fan only works on speed 4 then your problem is in the fan speed resistor which looks like this
I think you will find the thermal fuse has blown, they are less than a £ each, usually goes as the motor bushings are dry and puts a strain on things, take out heater motor and oil bushings properly. I will try and find the post I put up on the Ducato forum showing the repair in detail. https://www.fiatforum.com/ducato/335452-heater-fan-resistor.html