check/clean all the connections to battery. I did notice a cable that was not connected to anything. It's definitely a positive connector. Just to confirm the wires to the starter are the single purple(8mm) & (2)the double black/single red(13mm), assuming that is correct this positive cable doesn't belong to the starter. Here are the pics one standing over engine and one from drivers side.
1998 Grand Prix GTP 1.jpg