Can I install the tach from a suzuki boulevard M109 to a Suzuki boulevard C109,? they have the same motor I just need to know if the harness is there and if it will work... and also where do I start... where is the harness, is it inside the headlight or at the neck behind the plastic...

In my experience with Suzuki, they use the same wiring harness between modles such as the M109 and the C109 or DL650 and DL1000. More than likely you will find the plug in which to install the Tacho. Best have a good look at someones M109 and see where it connects, if they let you of course. These should be no reason why you can't fit it aftermarket anyway.