Tiderfish

I know this has been delt with before. I have seen the pic Icemark posted in the FAQ, but I do not know how or where to get that relay. The rest of the diagram makes sence to me. What are the numbers representing? If I use this setup, will I still be able to trigger the factory headrest amp to start up? If so do I just couple that with the 12V+ Blue wire from the new HU?

Thanks for the posts!

Matt|ttaM