You don't have to buy a new car to get a different radio. And if your car has a 3,5mm jacket (here we go! ;-), you could use the Chromecast Audio to cast to the car radio with your new iphone without that 3,5mm jacket.

You need a local wifi network for the CC, so you have to setup a wifi hotspot with your phone. Then you make the connection. The downside is that you cannot use car stereo buttons to skip to the next song. And you probably need double usb-charging for the CC and the phone. That's quite a lot of work to setup each and every drive, but for longer drives it might be a good option.