Lexus Lov'r

There's no button for it on the control head, and nothing in the owners manual either.

While listening to music, my volume will go up and down without my changing the volume, all by itself. It usually gets louder when I accelerate, and quieter when I slow down, but not always in that order, and sometimes louder than others! I've taken it in to have it looked at and was told that I need to get a whole new control unit/face. It was also suggested that there is a component that is responsible for this nuisance. It's a 2000 ES 300 Platinum Series. Any suggestions how to turn it off, if at all possible, or what the component is, if existant, or what can be done, if anything? It would be much appreciated!!