I have thought about doing this as well and thought about going to the local Pick-n-pull to find something GM & cut enough length of harness from it to splice in where I need it.
Edit. I have also purchased this from amazon for our boat. The radio is a pain to get to, so I mounted this where it has easy access so I can play/charge my phone through it.
https://www.amazon.com/gp/product/B0...e?ie=UTF8&th=1