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.
Aurora (Oldsmobile), Alero (Oldsmobile), Aztek (Pontiac), Astro (this name was used both by Chevrolet and GMC for entirely different vehicles. The Chevrolet Astro - also sold as the GMC Safari - was a minivan, whereas the GMC Astro - also sold as the Chevrolet Titan - was a Class 8 cab-over-engine semi tractor), AMC (now defunct car manufacturer).
Try your local library. That is where I go for automotive manuals, mine even lets me check them out. Another answer: Many auto parts stores carry Haynes manuals for most cars ($10-$15). I've found them invaluable when working on my own cars. The Haynes manuals cover almost everything you can do on your car, to include wiring diagrams, diagnostic codes (and how to read them), regular maintenance, brakes, emissions, all the way to engine overhaul…