I recently bought a 2013 C6 Coupe with an aftermarket sound system in it. To be honest, that almost stopped me from buying the car, but after inspecting the install and quality of components used (used to work in that business years ago), I bought it anyway. It's got a Kenwood head unit in it and all HUD functions seem to work as far as I can tell. Not having seen the OEM setup, I can't comment about losing Nav functions in the HUD, but everything else seems to work. The head unit has Apple Car Play, so I'm not worried about that function. As an aside, teh steering wheel mounted controls seem to work fine as well, so it may just come down to the quality of the installer.