1966 Barracuda new radio install

Hopefully this new radio doesn't draw a lot of power. If so, you may need to run all new wiring for it through the firewall. Otherwise, I would run the constant power wire (for memory?) direct to the fuse panel and attach it to an always hot terminal, such as the door or lighter circuit. Use whatever inline fuse the manufacturer recommends. Main power wire can do the same if you want to be able to use the radio without the ignition being on, or, add it to ignition 1 or ACC depending on your desires. I prefer to do it at the fuse box, but, if it doesn't draw much in the way of amperage, you can use the stock radio pigtail as your switched 12V source. There again, if it draws more juice than stock, I would go to the fuse box and use an inline fuse and not the factory one. Just add to the supply side of the fuse box, not the load side.