How do I rewire the power outlets in my 2015 Chevy Malibu so that they always stay on?

I wouldn't touch the existing wire harness, you can really eff things up by doing that. If you want an "always on" power outlet (even if car is off) pick up an "add a circuit" kit at any parts store and add another circuit to the interior fuse block and run the wire to wherever you want your outlet. There are two types of circuits in the fuse block, those that are always on and those that only have power when the engine is on and running. Use one of the constant, or always on, circuits if you want your power outlet to always be live. A circuit tester will help you identify a constant or switched circuit easily.

Thank you