There should be nothing wrong with old solder unless it is oxidized or dirty. The issue is to get the solder to wet both adjoining surfaces. Repair could be as simple as dabbing some flux onto the area and reflowing the solder. There's a tounge-in-cheek saying that "a clean joint is considered already half-welded". Heck, it may even work without the flux. The other caveat is to not overheat the electronic components any more than is necessary.

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95 E320 Cabriolet, 169K