First off, I’m a complete idiot when it comes to electrical/wiring... just so we get that out of the way when you read this. I’m replacing the old stereo the previous owner had in the car, which worked, with a more modern Pioneer head unit w/amp that I had in another car. The old stereo was only connected by the single power wire which has the “radio” label in the picture (it’s purple). 
Ive now connected that to the red power wire from my new head unit but nothing happens. I assume the issue is the yellow wire from head unit which also needs to be connected... I thought this yellow wire is the “always on” connection for memory functions etc. but I’ve also read that its for the ignition. Where do I connect this??
Also, is the Black wire that’s with the purple power wire ground? I didn’t trust it so I connected the new ground elsewhere.
Thanks. And again, assume you’re talking to a 5year old.