Tach problem
 

I have a tach problem on my 93 diesel. Now I know this is gonna be a dumb question but I
thought perhaps someone out there may give me something else to check that I have not.
The tach was on the truck when I bought it and has always worked , so says the previous owner. But now sometimes it works when I start it up and sometimes it dont. It is plugged into the factory plug for the factory tach. I have checked those wires there. Is there anywhere else I can check? Where does the factory wires run to? Thanks for any help.