If yours is severely scratched, and you want to put a new piece of plexiglass in, then this is for you. Do note however, that the trip reset lever will not be functional, but if it is like my car, it probably wasn't functioning anyway. If you are really picky about the reset lever functioning, go here to order a laser cut piece: http://www.customlasercutting.com/ or use a google search for laser cutting. I chose the cheap route--$1.00 at Home Depot using scrap the store had on hand. I didn't glue the new plexiglass back onto the housing. Instead it is sandwiched tightly between the two layers and does not move around.
After you thoroughly wash and dry the plastic, remove the protective plastic on the plexiglass' two sides. Sandwich in between both parts removed. These are the parts that you broke apart with the flat head screwdriver and/or utility knife.
Glue the tabs back together with a two part epoxy after you are certain that everything works correctly and that all of the surfaces are cleaned to your liking. You may want to clean the plexiglass with a soft, non abrasive cloth very lightly, then blowing it off with compressed air to get rid of the lint.
Then you can reinstall the circuit board and connect the wires to the car.
My tachometer's connection was a white and red wire. Make absolutely certain that your wires are labeled, as the wrong wire attached to the tach can cause it to burn up.