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Switch to the Bluetooth channel (Make Bluetooth the main multimedia source)
Car audio systems have several sources for multimedia playback, which are: CD/DVD, USB drive, FM radio module, Bluetooth etc. If you try to pair a Bluetooth device while the car's audio system is playing a CD/DVD, you will most likely fail.
Make sure to set Bluetooth as main a main multimedia source and then try to pair your device.
Contact the device manufacturer's customer support
Maybe your device's bluetooth issue can be resolved easily by the manufacturer's customer support team. All you need to do is to call them and follow their instructions. Also, most of the manufacturers have a troubleshooting page on their websites, which can help you fix your device's bluetooth feature by yourself.
Verify that the other device's Bluetooth feature is enabled and functional
In order for a Bluetooth communication to be established, the Bluetooth feature should be enabled on both the devices. If it is enabled on both devices, but a communication isn't established and if you verified that your device's Bluetooth feature works, check if the other device's Bluetooth feature works, too. For example, try pairing the device to another device.
Note: If you determined that the other device's bluetooth doesn't work, restarting the device should make it work again.
Maybe your device is not Bluetooth-capable. Check if it has a "B" icon, imprinted on its front side and check its user guide, too. Please note that some devices require purchasing additional Bluetooth dongle.