In examining ours, I found the failure is due to improper assembly line procedures - a very common situation in electronic fab, unfortunately. The practice is, solder the connection, then trim the excess lead. What happens is that the trimming equipment unavoidably puts a high shear force on the connection, breaking the alloyed metal. A hairline cylindrical fracture results, and the poor contact and arcing that occurs deteriorates the contact until it no longer functions.
Often these fractures are invisible, or nearly so. Oxidation of the metal may not make a simple solder reflow a good solution. The solder should be removed, and new flux used in remaking the connection. If there is no contact between the parts being soldered, that should be corrected at this time.
'91 MB 190E 2.3
'08 RAV4 Ltd 3.5
'83 Lazy Daze m'home 5.7