Normally it is not necessary to do something to automatically connect your cell phone to the car or other Bluetooth devices. With both having the Bluetooth system turned on, they should connect automatically, as long as they are close and you have already “paired” them before (the car bluetooth should appear in the “paired devices” of the phone).
Bluetooth Auto Connect
The Bluetooth Auto Connect application apparently makes the same automatic connection that Android already does as I indicated. However, it has other useful options, such as «Run App», which will allow you to open Spotify when connecting Bluetooth , deactivate Bluetooth after a certain time, conditioning factors to activate Bluetooth with calls or when connecting the charger.
Android Auto
An interesting alternative is to use Android Auto, an application made just to facilitate the use of cell phones and apps like Spotify when you are in the car. This app has two options: activate Bluetooth automatically when opening the Android Auto application and vice versa, that is, when activating the Bluetooth connection open Android Auto.
The ” Use Bluetooth ” option works like this. When you open Android Auto, the phone’s Bluetooth connection will automatically be enabled. Due to the automatic connection that Android already makes, as I mentioned at the beginning, it will automatically pair with the car, obviously as long as the latter also has the bluetooth on (the engine is on, or the switch is in the accessory position, which enables the current to the electronics).
This function also automatically deactivates the Bluetooth of the cell phone when exiting the Android Auto application, so you will not drain your battery by forgetting to deactivate it manually.
The Auto Start option does the opposite. When the phone is paired to the car’s Bluetooth, the Android Auto app will automatically open. In this case, the phone’s Bluetooth will NOT automatically turn off when you exit the Android Auto app. On this screen you could also deactivate the «Pocket detection», so that Android Auto opens without you necessarily having to take the phone out of the bag.
Shortcuts to activate bluetooth
If you want to facilitate the Bluetooth connection between your Android and your car or other devices with this technology, you can add a Bluetooth “button” to the home screen with the Shortcutter application widget. 
You can also add the Android Auto shortcut to that screen or even to the lock screen (Settings, Lock screen and Security, Shortcuts of the application), because as I said, Android Auto activates bluetooth when it opens and therefore it would connect with the car.
The last option is to go to Settings, Lock Screen and Security, Security lock settings and disable the option “Lock network and security”. This will make your device more insecure, but will allow you to activate Bluetooth from the status bar on the Lock screen.
References : Google Support
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