The installation kits made for the 96+ Taurus all include the harness. (The same kit is used for the Taurus wagon and Taurus sedan). This assumes you have the radio/climate control unit with rotary knobs for the climate controls. If you have the factory system with digital push-buttons for the climate controls, there's no kit made for it.
For these cars, I prefer the Scosche FD134030 kit: http://www.sjgreatdeals.com/sisfd134030b.html
Metra also makes a kit for this car, but in my experience it doesn't fit quite as well as the Scosche version: http://www.amazon.com/METRA-Ltd-99-5715-Taurus-Install/dp/B00029X1MS
Both these kits include the wire harness.