My stereo says 'muted' and does not work except when in reverse

They have a speed dependent volume so it does have wheel speed sensor input which could cause the wierd reverse only operation. It sounds like a software problem-I would try version coding the head unit(which will have to be done if you have a c.d. player or telephone in the car. Try to unmute it from the steering wheel buttons it may think there is a phone in the car and is muting the radio for phone operation. Your best bet is to pull codes and version code with a s.d.s. machine or at least a hand held tester but the majority of after market scan tools will not communicate with the d2b system which is your radio or sound system. good luck I hope I have been helpful