Using a multimeter (set in ohms to test resistance), test each terminal on the base of the plug to see which terminal corresponds to which speaker (left or right). You'll know they correspond when a value other than 1 appears on the multimeter.

Image 3 shows which part of the plug corresponds to each cable:

Left

Right

Common (or ground)

My results showed that the terminal with the larger hole corresponded to the right speaker, while the terminal with the smaller hole corresponded to the left speaker. Your results may differ.