Are you certain it was is working properly prior to the WP install? I had the same issue. The previous owner installed the WP and made a lot of errors. The tach/annuncator ended up being bad. I replaced it and that was the issue.
Other than that, there are only a few circuits that drive it, power ground and feed from crank sensor/ignition module. Validate the circuits one by one, If they are ok, then you may need a new annuncator ring.