2 possible places that you can tap into a tach signal.
Terminal 3 on the ECM connector, see yellow wire here
Or you can go to terminal 3 on the speedometer connector.
You will need a crimp on pin connector (P/N 74196-04) available at your dealer.
I have read that some people have issues with using pin #3 on the speedo connector on '06 bikes. (not wired between pin 3 of the ECM to pin 3 of the speedo connector from the factory?) So test it first before you make your connections final if you decide you want to use pin 3 of the speedo connector.
See this link for a tach install on a '06 Street Bob.
http://www.shareaproject.com/pages/p...,p,173,00.html