Does the tach display show anything or is it completely blank like the key is off? If it looks like the key is off, you could have a blown fuse, broken wire or defective display. If it always reads zero, I would suspect a bad sending unit or a broken wire between the display and sending unit. If you can find the sending unit, you can check it by unhooking the wires and hooking an A/C voltmeter to it. With the engine running, You should get a small voltage reading out of the sender that varies with engine rpm.