This applies to the traditional Gretsch 6120, Brian Setzer's signature model(s) (except the Hot Rod series), some Duo Jets, etc.
This applies to some varieties of the Gretsch 6120 model, some Duo Jets, etc.
Different pickups will have different color codes for the wires. If you wish to add a 4-conductor humbucker, consult the manufacturer's website for which wires feed from which coils.
In this diagram, the gray wires go to ground. Other colors were chosen arbitrarily.
Wherever you see a ground symbol next to a potentiometer, the wire should be grounded to the metal on the back of the pot. After that, all of the grounds should all go to one point to avoid ground loops.
For the sake of simplicity, the string ground is not pictured in this diagram (but you absolutely need to connect it!). See the "All About Grounding" page to learn more.
Note: The image above is larger than displayed. If you wish to print it, save it and resize as desired to take advantage of the actual resolution.