I tried to blue tooth my new iPhone 12 Pro Max to my 2020 GMC Terrain

Hello there, wanderingminds12. Thank you for posting your question to the Apple Support Communities. We’re happy to help you with connecting your iPhone with your car’s Bluetooth unit. 

To start, we’d like to provide you with several Apple Support articles that explain how to connect your iPhone with Bluetooth accessories and car stereo units. The steps in these articles should help you get both devices linked together: 

How to connect your iPhone, iPad, or iPod touch to your car - Apple Support

Pair a third-party Bluetooth accessory with your iPhone, iPad, or iPod touch - Apple Support

If your car has a CarPlay compatible stereo system, the following article may help you as well: 

Connect iPhone to CarPlay - Apple Support

If you are noticing an issue when connecting your iPhone to your car’s Bluetooth, please let us know what you are seeing exactly so that we can help you more. 

We hope this information helps. 

Enjoy!