I haven't had a Subaru cluster apart, but my guess is that the diode is probably a surface-mount type, which can be quite difficult to find and replace. It also might be part of a single assembly containing all the diodes, so it might mean replacing that module and not just the one diode. In any event, unless you are experienced with electronic repair work involving printed circuit board components, I don't think this is a DIY job.
Assuming that the other cluster is identical, other than the year, plugging it in would only confirm if the problem is in the cluster, which might be reassuring. Unfortunately, it won't bring you much closer to a fix, and I would be reluctant to plug in a cluster that might have its own problems, or not be completely compatible due to it being from a different year.
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