Problem with illumination in Explorer radio?

[i*like*beef]

I have a 99 Ford Explorer Eddie Bauer with the Mach audio system (steering wheel controls, 6 disc changer and rear controls) that I bought in August. When I bought it, the display on the radio didn't light up but the lights behind the buttons still work; all other functions of the radio still work. I bought a used radio on E-bay that was guaranteed to work but when I installed it it had the same issue. I purchased another radio that did the same thing. I have a hard time believing that 3 of 3 radios all have the same problem; I don't think switching to an after market system will fix me. Is there a wire in the wire harness that controls the illumination? If so, which wire is it and where does it go from the wiring harness so I can track it down. I know it's dumb, but it's driving me a little crazy not having the clock and it's a pain trying to find a good radio station.

Thanks in advance for any help you can give!