It's not the connector exterior that causes issues, it's a buildup of corrosion between the connector and its mating surfaces on the interior. Corrosion doesn't conduct electricity well.
Best for push-on connectors is repeated mating and removing with electrical contact cleaner available at an electronics repair place, with possibly some tightening up of the mechanical connection by gently bending the bits that grab terminals.
For bolt-down connections, nothing beats wire brush, anywhere from 80 to 150 grit wetordry (emory), sometimes even a small file. When you can clean both (top & bottom) surfaces of a ring connector, plus all flat surfaces around the bolt/stud, plus the stud threads, that's ideal. (Access may limit what you can do, of course).
On my GolfIII::
On both ground and power sides, I found significant corrosion on the TCU and ECU connectors, corrosion between the ECU, its carrier, and the plenum where it mounts, bad corrosion on the engine grounds, bad corrosion on the ground strap from coil to head... though you won't have the latter.
Modified by 95GrannyGolf at 6:47 AM 5-20-2007