G'day Jerry, i have found that CRC / WD40 takes off the gluey adhesive residue left after removal of things (like stickers), not sure if it will attack the old contact though. try it on a test patch.
have you tried a range of solvents (Metho, acetone etc) ?
like Chrisdespo mentioned, try the citric acid products (Orange Power brand "sticky spot and goo dissolver" from supermarkets is a good one) maybe try eucalyptus oil.