Hello Juneville,
Thank you for posting to the Apple Support Communities.
You may want to check the audio balance settings for your car and iPhone to be sure that audio is not being sent to one side only. The following Apple Support article explains how you can check your iPhone audio settings, you may want to contact your car manufacture for help with the stereo settings.
It may also help to pair your iPhone and car audio system again from the Bluetooth settings of your device: Pair a third-party Bluetooth accessory with your iPhone or iPad.
If you continue to notice an issue, please test playing audio from other sources in your car. It can also help to test a different Bluetooth speaker with your iPhone.
Please reply back with your results.
Cheers!