I wonder if there is a common place for a wiring fault to occur in these cars that kills the function of the license plate lights.
I have removed the harness which attached to the trunk switch and these wires seem OK and the trunk switch works. I have inspected the harness that connects to a connector near the left brake light unit and those look undamaged externally. This harness has a hard plastic tube and some pretty stout looking insulating jacket near the trunk lid hinge that appears undamaged, and i dont know if i could properly repair it if i started cutting into that type of insulation, and it does not appear to have been damaged.
Am i looking for a broken wire inside those things, or would I have some clue of that by simply inspecting the plastic sleeve and insulation jacket on the harness
without cutting into it?
So far i have only checked the voltage at the bulb sockets and fuse #31 in the passenger compartment. Fuse is good and bulbs do not light.
What next?
It's a 2004 S60 AWD

 

Turned out that the lights worked when I closed the trunk and shut off when I raised the lid.
Broken wire was acting as a switch. Fiddled with the harness and determined that the broken wire was in the harness after the hard plastic sleeve attached to the hinge and just before it entered the trunk lid sheet metal at the grommet where it bent repeatedly upon open/close of trunk. Sliced open insulation and made the solder repair and taped it all up. Fixed.

Now maybe I won't get pulled over for such a dumb thing.