What exactly is required for Apple's Find-my-car feature to work?
Yes, you must have a CarPlay device &amp; the iPhone must connect to it when you get in the car &amp; disconnect as you leave.
That is the trigger to update your Parked Location.

It works over either direct connection on Lightning cable, or using Bluetooth, though if your CarPlay is set to Lightning, it will not update if you have Bluetooth on, but don't manually switch to it - CarPlay does not 'auto-swap' to Bluetooth if you don't plug it in.

Apple documents this here:


https://support.apple.com/en-us/HT207227


The location privacy can show you frequent locations as well as parked locations, so if you have disabled that location tracking, you could expect maps to not mark your Bluetooth disconnect locations. Also, not pairing to Bluetooth would likely prevent is feature from working 

(Information source in addition to above is personal experience &amp; wondering how the heck it sometimes worked &amp; sometimes didn't ;)