On our 2018, there's a window, and door lock lockout directly under the driver's window switches. IIRC, there's a little light on when the lockout is on. Check that. Otherwise, the upfitter may have gone in and actually pulled the back door panels and unplugged things. Some still do that. Our 2012 Caprice 9C1 came like that, even though there's a window plug at the base of the front passenger seat for that.