By using this custom harness, it allows you to plug the existing 1978-1988 G-body radio harness and antenna lead into this harness, which plugs into the later radios.
This eliminates cutting any of the factory wires. It allows the 12-pin harness to adapt to the 21-pin harness in the later AC Delco radios. If you have a power antenna option, it will still work.
The harness and antenna lead can be seen in the photo. The big plug to the left is what the stock 3-part harness plugs into.
This is the same harness we have put with every late-model AC Delco CD player we have sold over the years.