First off the Deuce did not come with a tach, it is an accessory item. The connections for it are, therefore, somewhat arbitrary in that the installer might chose to find a power supply or ground point other than the one recommended by Harley. That said here are the connections. First there is power supply and the connection for that should be a aplice into the main harness made where the main harness passes near the rear tank mounting point. The ground wire from the tach is often attached at the rear tank mounting point as well, it will be a black wire attached to a bolt.
The next connection is the tach sensing wire. Harley did not put a tach on the Deuce but they did put the wire in the harness in case you wanted to install one later. Remove the dash panel, which is held on by one screw in the front and two bolts on the back, and you will find a blue wire just forward of where your keyswitch mounts. That is the tach sensor wire and I think you will find that your connection is bad.