You bought the car used. I would pull the instrument cluster and have a close look at the back side. The tach relies on a ground connection that travels the length of the cluster but originates on the speedo side (strip of copper on the plastic backing of the instruments). In the event that someone had rolled the odometer back or otherwise disassembled the cluster before you purchased it, they may have messed up the ground connection. Clean all copper strips you see back there and pay particular attention to places that are held together by screws.