My stock tach was stuck at 0 RPM when I bought my car. I very carefully took the tach apart. It wasn't very difficult.
You can break it down to just the tach face, meter movement and the printed circuit board. Look for any debris in the meter movement (area where the needle attaches to the tach - it should be some springs and maybe some tiny tiny bearings) and blow out any crap that might be on the meter. Just be very careful. The springs that operate the meter needle are very easily bent out of shape. Also, make sure the needle isn't sticking onto the meter faceplate. Get a small brush and some rubbing alcohol and clean everything up. Using your finger, very carefully exercise the meter by running the needle up and down a bunch of times. That all seemed to work for mine.
