It is possible to use a Bluetooth dongle with a car stereo's USB input. The problem is that most Bluetooth dongles are not designed for this and therefore won't work. You'll have to find a Bluetooth dongle that is specifically designed for non-computer USB inputs. These are usually labeled "car stereo" or "home stereo" or other terms to indicate they work without a computer. When you find one, however, it's a bit of a gamble if it will work or not since every car stereo's USB input can be a little different and every Bluetooth dongle is a little different. You might have to try 2 or 3 different Bluetooth dongles before you find one that works with your car stereo.
Here's an example Bluetooth dongle that MIGHT work: https://www.amazon.com/dp/B074WVJT6L
If you can't get a Bluetooth dongle to work with the USB input, the next-best options would be an auxiliary input cable or FM transmitter, like the other answers mentioned. The aux cable will give you the best sound quality, while the FM transmitter might be a bit more convenient.
The problem here is the limits of your cars stereo system. Your Bluetooth dongle needs a driver from the system as it is not designed to offer this function you can't get it to recognize the bluetooth or set it up.
You'll need a device that has the smarts to do this in-between your phone & cars speaker system (via radio or USB)
An old phone hidden inside the glove box could do this. Pairing the phones via bluetooth: Radio > USB > Old Phone > BT > your Phone.
The other option here is getting a new car radio which has native Bluetooth
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