Speedometer help

[HottScennessey]

hey,
I just bought an 88 supra turbo about two weeks ago, its great, new engine, nice solid car. I'm not very familiar with this car yet, but i'm getting there. when i bought this car, the speedometer didnt work. the guy i purchased it from told me that the short cable behind the dash was broken, so i took it to a shop. the shop was driving down the road with my speedometer working, and all of a sudden, it stops and drops to zero again. they say that the long cable in the back is broken now. I have them replace that, a couple hundred dollars the speedometer is still not working... i have another speedometer that the guy i bought it from included with the car, but when i installed it, it still didnt work. Is there anything i should look for, any moving parts behind the dash/speedo? I'm not sure what else to try. Any help would be appreciated.