Little fasteners hold the plastic bezel in place. You don't need to pull the dash to fix the tach. First check the pink wire that goes to the tach. That wire can sometimes have a fuse holder in it. Check that fuse. With your volt meter and with the car running, check the pink wire for 12 volt. There are only two wires that go to the tach. The pink one with 12 volts, and the brown one that picks up the pulse from the distributor. The tach gets its ground from the cluster housing, so you could run a temporary ground from the nut above the connector terminals (that's where the tach is actually grounded) to somewhere on the body of the car. However, first check that fuse. I have a document on how to remove the dash, if you want it send me your email address. It will tell you where the dash is connected. I also have a document on how to pull that cluster if you can't figure it out.
On the ground, if your cluster lights work, you're grounded. However, it's always possible that the ground circuit could have broken before it hit that ground screw and nut for the tach.