Tach not working.

[BC69]

My car has an aftermarket covans dash insert, and autometer gauges. Lately the tach hasnt been working. All the other gauges are working just fine but the tach isnt budging. I think at one point when I turned the car on it would move over to the 0, but for the most part it remained 90 degrees down and nothing. I thought it might be a fuse, but all the other gauges are working. A little work had just been done recently on the transmission and starter, could anything have been knocked loose? Any ideas...another one of those simple problems that I havnt yet found the simple solution to.

Thanks Again

Tim