Well, this turned out to be a royal pain!
The car had been to an "Alfa Specialist" with this problem, and it was diagnosed as a motor ... but not repaired as they did not want to do it. Talk of dropping roof lining and other issues ... mumblings and general impression they really were out their depth ...
So the first thing was to prove the switch, as I was not convinced by that previous information or the reason for not doing the work.
The switch was dead, not one position worked ... and the motor supply was missing, no positive, simple, it'll be the fuse?
Luckily a secondhand switch unit was found quickly, so test it on the bench, and all appears good ... connect it and start the power supply tracing ... start with the fuse .... it's good! Darn!
It's located in a separate fuse holder group, not on the main interior board (behind/above it), and it's tricky to access ...
Ok, so with fuse still removed, use the Powerprobe Hook to power the motor supply but only after dropping it out for access to the connectors (had to undo 3 bolts but it drops out the hole for the control switch - yes the roof liner can hapily remain in situ), and motor works disconnected but not when attached to the sunroof, so guessing it might all need a bit of a lube and clean of the runners/seals ...
Sunroof sliders cleaned and motor still not working correctly, looks like the motor is just past it best, the roof is not needing excessive effort to operate it by hand, and the PCB looks fine, no heat or burn marks ... maybe this took out the switch? Anyway luck is shining, a New motor, showed up perfectly timed, on ebay ... bargin grabbed.
That just leaves the supply issue, check the power side of the fuse, good battery voltage, happy to supply 10amps with no real drop. Probe the component side (motor) and also battery voltage, oh, still got the PP Hook inserted, turn if off and voltage gone, test wire to motor and appears fine, again no excessive voldrop, so though of broken wire or short wrong! Try to insert fuse and it's not happy ...
It's too hard to see, so use the old camera trick, took a picture of the fuse holder and zooming in, it's clear to see ... the fuse had been inserted down the side of the holder, not into the contacts and had managed to make no contact on 1 side ... bending the socket slightly making the correct insertion hard. Fortunately found this before stripping out the fuse unit, oh the joys of zoom on camera's! A fiddle with a screwdriver and fuse now happy, motor happy, sunroof happy.
I guess the poor car had undergone a few previous attempts at repair, and none were more that half hearted, how three separate faults managed to acumulate is probably not coincidence, clearly the fuse is technician "ham-fisted" error, the motor/switch might be related, or might have been someone poking and powering things they did not understand, who knows!
So sadly no scope captures, sorry, but a happy wife and summer coming so hopefully the darn roof will get some use!
Thanks STC for the wiring, without it the switch diagnsotic would have been impossible.