It's been my experience that a lot of the Harmonix hardware from that era have cold solder joints. So the cable is less likely to be the failure point, but the part that stinks is that there's no way to distinguish bad solder visually. What I ended up doing was taking a soldering iron to the larger joints (and all the ICs) and reflowing them one at a time.
If I'm not mistaken, the buttons on a Guitar Hero controller do correspond neatly to other buttons on a standard controller, so you might be able to use some other game with known inputs. Probably putting a USB adapter on it and using the game device input on your PC would provide more definitive results. What console is this axe for, incidentally?