It is a newer model, bought a few year back. I found the booklet for my other Sun Super tach and it says run it to a positive that only gets power when ignition is on, and to engine or chassis ground, so having it run to the dash may be the problem. It was on the negative side before and didn't work there either.
If anyone can remember where you ran tour wires ( e.g red to fuse block accessories terminal)please post, it would be helpful.