Tach's are one of the first gages to fail but you should always verify that the tach is bad before ordering a new one. The tach will have a ground wire-black, a blue wire for the backlighting, and a gray wire from the Ign. coil. and possibly a purple if it has a built in hourmeter. If the backlighting comes on with the nav. light switch your ground is good. Now you need to verify the gray (think of it as a sender wire) is good, it has a long way to go & several connectors. I disconnect one end and run a temp long jumper wire from the coil to the tach, if still nothing (with the eng. running) replace the tach.
There are several out there call me or send an email for a replacement quote....RonT