I had a similar issue with my Pixel phone. It turns out the minimum Bluetooth AVRCP version has been updated and I could no longer pair or connect to my older car stereo. Android has a setting in the developer options to change the AVRCP version and after I changed this to an older version the pairing worked again.
Unfortunately it doesn't look like iOS has this setting so you probably need a new stereo. You can also buy a new Bluetooth audio adapter for your stereo that supports the newer Bluetooth standard.
I had a similar issue with my Pixel phone. It turns out the minimum Bluetooth AVRCP version has been updated and I could no longer pair or connect to my older car stereo. Android has a setting in the developer options to change the AVRCP version and after I changed this to an older version the pairing worked again.
Unfortunately it doesn't look like iOS has this setting so you probably need a new stereo. You can also buy a new Bluetooth audio adapter for your stereo that supports the newer Bluetooth standard.