Is the issue that you want to use your iPad in place of an iPhone to supply data to an existing CarPlay electronics head in your car? If yes, then the issue is really that CarPlay doesn't have support for iPad. The only way that you could get around that is for your iPad to 1) identify itself to CarPlay as an iPhone model, and 2) have the necessary software frameworks on the iPad to communicate with CarPlay. I think that the odds of either of those being achievable are pretty slim, and taken together almost impossible. If it were achievable it would certainly require jailbreaking.