InCar Express wrote:

When the ignition is turned on, the 0.2 and 11.5V become around 0.5 and
5V respectively - the 5V line stays at 5V for a couple of minutes after
the ignition is turned off, before returning to 11.5V

Is all this normal?

I'm fairly sure that Fiat use CAN BUS control instead of an ignition
feed on this unit, so the above is probably indicative that everything
is OK. Unfortunately though, there's no simple way to confirm that the
signals are OK if they are using CAN control. You may well have to
take the car to Fiat for diagnosis.

The manual for the unit does indeed show that there are two CAN pins.

I've tried the unit connected to another car: it doesn't power up. The
stereo from the other (older) car works OK in the FIAT (so long as I
take the 12V battery line to the 12V ignition pin, because the FIAT
doesn't supply the latter).

I'm presuming (hoping) that it's the stereo that's dead, rather than the
car!

Daniele