How do I wire a new switch on my 15 hp gravly what wire to where

Hi,
Not all Gravely mowers have the same wiring scheme. Gravely is owned by Ariens and they have a wealth of information on their website. Manuals at:
Gravely (http://www.gravely.com/)
But you must have your Model and Serial numbers as there are feature breaks within models that are determined by serial number.
Alternately, Indak makes almost all switches for the industry and they are either labelled on the back plate of the switch or on the blade connectors on the back. Such as B for battey, G for ground, etc.
Peace,
Clarke