You should just be able to tap into the line and run it off in parallel. You basically want a relatively high impedance input in your measurement device to not interfere much with the normal operation, like with an Oscilloscope or DMM in voltage mode. If you can figure out the impedance of the game-cube input,it would help, but that could be complicated in itself (It probably is not just the DC resistance, though that is a good place to start).