For sale are adapters to use a aftermarket amplifier in the 2017+ axis and Malibu. The factory amps have proprietary plugs where typically RCAs for input and bare wire is used for the remote.
I have made these plugs that make switching amps a breeze. Plug and play.
The eBay listings are mine and there are 3 separate listings. 1 for the tower adapter, 1 for the inboat adapter, and one to repurpose your factory amp.
Tower:
https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.com%2Fulk%2Fitm%2F233324228899Inboat:
https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.com%2Fulk%2Fitm%2F233324231096