panasonic cq-c1300u, no sound

I just bought an old car with a modern stereo, a Panasonic CQ-C1300U if it matters. The stereo turns on and reads CDs. However, it makes no noise at the speakers. Any time the stereo is on, there are 12-13 mV and 5-6 mA, steady, at the speakers. No voltage or current when the stereo is off. I attached a known-good speaker and got no noise. Can anyone explain how to continue the diagnosis of this problem?Thanks!