Where can you get a wiring diagram for the stereo in a Chevrolet Astro van year 1999?

Go to your local parts store and ask them for a Haynes repair manual for the year and type of your vehicle. It costs around $17.00, and although it is not the best, there is enough info in there to get your stereo wiring figured out. Chiltons also makes manuals, plus there is always the dealer. Sometimes even the dealer will print out a page of schematics for you. It just depends on their generosity.