Okay, just to be sure here, the connection is okay, it is just that the volume is low in the vehicle and the microphone is a problem in AirPods and other headsets.
I'm not sure about the vehicle setting, but do you try turning up volume with the car? I can remember in the past if the volume on the phone was really low, it would interfere with turning up the volume in the vehicle. Kind of like when you would be using an external speaker and the gain on the speaker was not turned up. I'm looking to see if I can find anything about the microphone issue, but I know there is a new setting in iOS 26 I've not seen before about the microphone. If you go to Settings>Sounds & Haptics, there is a setting for Input. It defaults to iPhone Microphone. It you are using the AirPods it should select that. Make sure it is and is not trying to use the iPhone microphone that could be a distance away from you or in a pocket that would affect the volume.