Heater on fiat ducatto only works on maximum setting.how to fix??

I assume you mean the heater fan only works on maximum speed?

Usually this signals the heater speed dropper resistor has failed, though it could be the switch or wiring. The resistor providing the lower heater fan speeds is designed to get hot and so is often buried in the heater unit cool air intake. If the motor starts to become tight and run slow or consume more current or anything restricts the flow of cool air the resistor can soon fail.

I hope this helps.

That sounds like fan switch you didn,t say what year some are vacuum operated will require finding connection under dash, if electrical just change switch