You're in the right section. On pages 8A-150-1 and -3 the left front speakers are shown for the different radio options. In both diagrams, splices S510 & S512 are shown. Flip ahead a couple of pages to the component location (pages 8A-150-5 & -6) and you will see the text description "composite harness, in front portion of LH door" and a figure reference of 201-15-A for the splices. Now turn to page 8A-201-15 and you will see a drawing of the locations.
I think the splices you are looking at are S242 and S298 which split off and go to the amplifier. The location for these is "I/P harness, below front of center console". I suppose there could be a broblem here as well but since both front components are turning off at the same time, I would expect the bass to the LR speaker to go out too. This is just an assumption because there was no mention of the other speakers cutting out.
What vehicle are we talking about exactly?