If the switch is open circuit (ie ..it cannot be switched ON ) then if you are OK with electrics you could just short it out so that it is ON all the time the ignition is switched on. You can control the volume from low to high.
Or if you cannot find an identical switch to replace the faulty one, you could wire in another separate ON /OFF switch.
A bit messy though. Perhaps easier to buy a replacement off Ebay
John
September 2008