If the radio has a knob that you turn for the volume, it could also be a bad potentiometer in the radio itself. The way to determine if it's the radio, or the wiring to the speaker, is check the volume of the same side speaker in the rear when the volume has gone down. If the volume is low in the rear at the same time the volume is low in the front, I would guess it's the radio, unless the wiring is in intermittent short, in which case the symtom would be the same.
"To compel a man to subsidize with his taxes the propagation of ideas which he disbelieves and abhors is sinful and tyrannical."-- Thomas Jefferson