Radio installation problem

I am trying to put a new Koss MS2015 radio/cd player in my 1992 Miata. I have connected the power leads to a 12 volt pair on the car but the radio does not power on. The ground is negative. The wires from the car are hot (checked with a meter) and the connections are firm. I have pushed the reset button, all to no avail. So far as I know the radio is not defective. I brought the first one back thinking that maybe it was. The only thing I can think of is the lack of a ground wire to the frame of the car. But doesn't the black wire provide a proper ground? I have not at this point attached the speakers or the antenna. But the unit still should power up without them, no? Appreciate your advice