One of the weak points of Fiat rear light clusters, most of them
are riveted contacts. I have on several occasions soldered wire
links to bypass the joints.
The PCB's seem to stand a soldering iron, one with generous
thermal capacity helps, the fittings and board act as
heatsinks which cool lesser capacity irons so the solder doesn't
flow. This is a permanent cure if you get it right.
This assumes you can judge the temperature and not cook
anything in the process, or of course the nearest breakers
yard, those these are getting increasingly expensive, long
gone are the days of climbing over unstable stacks of
cars of a Sunday morning to get the bits you wanted,
alternator a 10'er to you mate.
Also check the grounding points, they give problems as well.
For the edge connectors, Duraglit or Brasso on the
PCB tracks, then apply Vaseline.