I got a seemingly plug and play (pray) aftermarket headunit off of Amazon. Pretty straightforward. In theory, all you have to do is remove the radio, swap the vents over and then plug up everything. Unfortunately, it only gets power when I join yellow and red together, resulting in a constant on scenario. I have checked the fuse (and replaced) in the back of the deck. I've checked all related fuses under the dash and hood. Using a multimeter, I get 12v on yellow but nothing on red, even with the key in the ignition. What am I missing here?