Odometer woes...

I have a '98 convertable V-6 'stang and the speedometer went out in it (the odometer stilled worked). I picked up a used instrument cluster on eBay and installed it. Everything worked great except the odometer.

So I thought I just got a cluster with a bad odometer. I purchased ANOTHER cluster on eBay and when I installed it the odometer didn't work in it either. Now I'm getting suspicious. Does the speedometer and odometer use the same signal to operate? Is there something in the car that only allows the original odometer to function? Any help would be appreciated. Or am I just having bad luck with instrument clusters?