My 94 runner came to me minus the radio, but did have an amplifier between the vehicle harness and the radio plug. It's also missing the main rear speakers.
looking at a schematic for the system, it appears as if the rear speakers are self-amplified. Can anyone confirm this?

 

Yes, the rear speakers in my 91 4Runner had amps in each of the speaker enclosures. When I upgraded my audio I installed better speakers in the boxes and bypassed the internal amps.