I checked the speedo cable that is used in the 81-82 Luvs and the 82-87 Isuzus, and I have one that is still in the factory package. The package is unopened, the part numbers are GM # 94253993, the Isuzu # 8-94253993-2. These are stamped on the plastic "envelope", another possibly generic part number stamped on the outside above the parts tag is 5569 000.
I'll sell the one I have for $45. plus $10 shipping, I just paid that much for a special cable to fit my 78 Luv, T 350 to a 5/8" aftermarket speedometer, so I think that's fair.
Since it is still sealed I don't wish to open it, but if you want a picture of it since you can easily see both ends let me know,
pinkertondusty@aol.com