CD/radio head unit problems - suggestions on how to diagnose?

The Visteon CD/radio unit in my 2002 FIAT Stilo failed to turn on with the car the other day, and has resolutely failed to do so since. No
lights come on, and there's no other sign of life from it.
The appropriate fuse in the fuse panel is OK, as is the fuse on the back of the unit.
I checked the power reaching it from the cable loom. There are four connectors, which deliver approximately:
0V, 0.2V, 11.5V, 12.5V.
When the ignition is turned on, or the door is opened and the courtesy lights come on, these change (until about 30 seconds after the courtesy lights go out, when they return to what they were before):
0V, 0.5V, 5V, 12.0V.
Is all this normal?
The part number is Visteon 2FCF-18C838-BE, or FIAT 735296997.
I've dismantled the unit but I can't see any obvious signs of failure on the main board, but I suspect that's where the fault lies.
I'd be most grateful for any help.
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