If you go to www.themustangshop.com there are wiring diagrams for various years. The one for '68 is kind of hard to read, but it is better than nothing. These diagrams are .jpg.
http://www.moby302.co.za/wiring/diagrams.html
veryuseful.com/mustang/tech
When working on anything electrical in a car, it is imperative to have a diagram. A diagram of the cooling fan wiring to a 1996 Mustang can be found in its maintenance manual.
You will find a free wiring diagram at: www.veryuseful.com/mustang/tech/engine#Diagrams Good Luck!
Car stereo wiring color codes for a Mustang can be seen on a diagram. The diagram may be found in an owner manual or request from the stereo company.
Classicarwiring.com (eBay)
Try MOTORLIT.COM.
http://www.veryuseful.com/mustang/tech/engine/
wiring diagramfor 1968 windshield wiper motor
Here are the 66 wiring diagrams: http://www.hammar.dyndns.org/wiring/wiring1966.htm
go to your local auto parts store and buy a haynes manual. In the back they have full wiring diagram.
Diagrams can be used a guide, when making changes to a car. A free wiring diagram can be obtained at an automotive part shop.