Radio wiring diagram needed

You should have three 4-pin plugs as shown here. The fourth 2-pin plug wasn't used until the mid '80s on radios with a digital display. You won't have that plug, but you will need the constant 12-volt feed to maintain the clock and station presets on your new radio. You'll need to run that wire from the fuse box. Most GM fuse boxes had accessory terminals marked in them. Look for one that says "12V". Another one will be listed as "12V Ign", or just "Ign". You don't want that one. You already have that switched 12 volt wire in the connector.