[hollander]

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.

[SysArchitect]

This still has the issue of not being able to use your car controls to control the music, but:

https://www.amazon.com/Antec-Portable-Bluetooth-Receiver-Hea...

Pair this with your phone, and plug it into power. When your car turns on, it powers on, pairs with your phone, and spits audio out over 3.5mm to the car stereo.

Comes with a 3.5mm male -> 3.5mm male plug for this purpose.