The VOL+ switch signal goes direct into the radio controller. You may have some water induced corrosion in side (the rubber buttons tend to leak eventually). May be worth a shot at opening the controller up and inspecting it. Plus you can check the switch function at the controller. You should measure continuity from pin 9 (Brown/Blue) to pin 11 (Blue/White)for VOL+ and from pin 9 to pin 12 (Blue/Red) for VOL-.