Hi, and welcome to the Apple (user-to-user) Community.
Sorry I'm no expert in this area, yet your request surely deserves a response:
Re: Tesla
Tesla's Motor club discussion:
https://teslamotorsclub.com/tmc/threads/bluetooth-not-connecting-automatically.252189/
says:
“Bluetooth now connects to your paired phone only after you are sitting in the driver’s seat and all doors are closed”.
and re: reconnecting:
The (non-Apple) discussion below has an explanation and offers a solution
https://electriccarpledge.com/tesla-not-connecting-to-phone/
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Re: Apple & iPhone:
Wondering if updating your iPhone's iOS to the newest available can help:
or
Whether following the steps in Apple support's article (link below) can help:
How to connect your iPhone, iPad, or iPod touch to your car - Apple Support
Else: troubleshooting:
If CarPlay isn't working as you would expect it to, find out what to do:
If you need help with CarPlay – Apple Support
All the best :-)