Where can you find a stereo wiring diagram for an '89 Toyota pick up?

Chilton or Haynes Auto repair manual, available at your local library. Its pretty straight forward, like most factory installs. If your adding an after market unit, you don't want to use factory speaker wiring. It's the minumin required, to power those cheap, paper speakers, installed at the factory. Same holds true, for the +12 volt pwr, going to the factory radio.