This digresses a bit but by any chance is your phones volume not set to max when connected to your car? While some Bluetooth speakers will synchronize their vol

This digresses a bit but by any chance is your phones volume not set to max when connected to your car? While some Bluetooth speakers will synchronize their volume with whatever your phone's set to, most just treat it like an aux cable and will have its own separate volume. So even if your car speakers are set to max, if your phone volume is at 50%, then it's not gonna be that loud (and will sound like shit).