Some amps, like the widely used Rockford Fosgate PBR300X4, don't need a remote turn-on. They turn on by sensing signal on the input channels.
If you're using a different amp that doesn't have that feature and requires a remote wire input then any switched +12V source will do. One I know of under the fairing is the +12V line in the unused GPS input plug. I'm sure there are various others.
2012 Victory Cross Country - Imperial Blue
2008 Honda CBR1000RR - Winning Red
2003 Honda VTX 1800C - Illusion Blue