I can restore them cosmetically but not functionally. I can however replace the entire interal unit with a new reproduction if one is provided (I don't keep them on hand) and that will usually get them working again. Downside is that the reproduction units are weaker than OEMs so that can be a PIA. I can also use the internals from any other NOS unit (different number etc.) as long as it is configured the same.
I think someone with more experience doing electrical type repairs (alternators, etc.) might be able to repair SOME of the originals depending on the issues they might have but I don't know of anyone offering that service.
Generally it's just that the spade connectors have become detached from the (very) fine copper wire ends (there are two ends, one is soldered to the lower spade base and one is soldered to the round nickel colored washer at the opposite end). To access the inner workings you need to un crimp the crimped end which isn't too difficult with some side cutters or other prying tool but you need to be careful not to cause collateral damage. Once repairs are made you can add some JB Weld to the end around the washer to keep it assembled as opposed to re-crimping.
