I have a 2018 RT standard. When I play XM radio via the speakers via BT I cannot get max volume except when going over 45 mph with the auto volume on high. I would like to get max volume going slow and cannot figure it out how to do this. The auto volume has low, med. and high. I would think if I could shut this off I would be able to control the volume to max even at a complete stop. Pls. advise and pre thx.