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Given the age of the relay and some properties of solder, I would categorize this simply as solder fatigue.
It's a well known phenomenon and there are several mathematical models to predict it. When I looked the solder joints over with the microscope I was expecting to find cracks before I actually did, but they can be hard to impossible to see with the naked eye.
Even at room temperatures, low melting temperature metals (like solders) exhibit what's known as creep deformation. This speeds fatigue up quite a bit and so do thermal cycling and vibration.
In an environment like a car, 25 years before developing cracks IMHO isn't bad at all (many a CRT TV failed far quicker caused by cracked solder joints), and the fix is simple.
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1992 245 Polar B200F M47
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