My 4Runner factory rear speakers had built-in amplifiers which required the additional power wires to each speaker. I seem to recall extra wires at the radio which were for the different factory options (cassette player, equilizer, etc.). Sorry, I don't have a diagram handy.

'86 4Runner SR5 - 22R-EB with V-6 Throttlebody, 2" 4Crawler B.L., Ball Joint Spacers, Rock SliderZ, TJ Coils, 5.29's, Rear Locker, 33" Duratracs