Plus and minus signs on the multifunction steering wheel. Car music players basically adjust the volume manually. There are usually two knobs under the central control screen. The left knob is for adjusting the volume, and the right knob is for adjusting the song track. Some models also have buttons to adjust the volume on the multi-function steering wheel. Mr. Walter Owen Bentley, the founder of Bentley, initially pursued the concept of ultimate excellence in car manufacturing.