This should get you started. What you will need to figure out is if both 12 Sockets are powered by one fuse, or separate, which will dictate if you need 2 relays, or one.
You will also need to find what circuit to tap the yellow wire into. Using a test light, test the fuses in your fuse panel until you find a suitable one that powers up with the key in acc and run.
I put a picture of two ways to integrate a circuit into your fuse panel. I prefer the add-a-circuit style for higher load items. For this low current application, either will work. If using the "fuse tap" style, be sure to tap into the fused side, not the constant power side. You can also use a "scotchlok" connector tie into a wire that is keyed, but I am not a fan of this style splice. If you choose to, the stereo would be a good one to tap into.
The relay source may have 5 terminals, the extra one will be 87a. It will be a "normally closed" terminal, and you will not need it. Just leave it blank.
This is only one way of doing what you are asking. This is the way I would choose to do it, as it alters the least amount of factory wiring, and is easy to reverse if you so choose.