If the humming is in all four speakers it could be the stereo itself, if only in one speaker perhaps it is the speaker, (try substituting the speaker with another). Is the humming only when the engine is running if so it may be a problem of interference form the alternator.
Plug wires...any kind of open in the plug wires would cause EMI. electro magnetic interference in the sound system. Replace the plug wires and if that doesnt solve the problem then go to radio shack. they sell whats called a radio choke. it has a capacitor that absorbes the fly back volts should quiet the humming noise down.,