Here is what you would learned had you read the "Before Turning the Game On" section of the PinRepair guides
2b. Before Turning the Game On: Plug Connectors Cleaning & Dim/Bad Lamp Sockets
Before plugging the connector in, take your 400 or 600 grit wet/dry sandpaper and sand the circumference of the male side of each pin of the plug. This is the area that the female plug bites in to. Wrap your sand paper around each pin, and rotate a few times. They don't have to shine like a new penny; just get the major crud off.
Alternatively a small wire brush can be used (both in the long and short directions of the male connector pins) to shine up the male pins. (Personally I use a wire brush instead of sand paper as it's quicker.) This works really well. Wire brushes are available for about $2 at Home Depot in the welding department (get the stainless steel variety, not the brass brushes.)
Gottlieb Coin Door Connector.
It's also a good idea to clean the connectors that attach to the bottom panel of the game, and the coin door connector. Gottlieb coin door connectors are especially important - HERE'S THE GOOD PART: If this connector is not making good contact, the game will refuse to work, or some features will not work!