I was just wondering if there was a way to hack/ jailbreak CarPlay head unit to be able to use non Apple approved apps on the CarPlay screen on my cars head unit?

Not that I've managed to find yet. The only "hack" I've managed to find is ignition to fool an ipad to look like it's running car play. I haven't got car play activated so haven't bothered exploring it yet. It does seem to allow some non carplay apps to run but wasn't quite what I ( and I suspect you) we're looking for.