Welcome!
I'd lose the tach; just use your heart.
As HB says, it's the single indicator bulb that flashes along with your signals. If you simply remove it, the problem should go away. If you want to correct it, you'll need to build a diode bridge. If you need help with that, let us know.
Try to find some images of what your bike looked like with plastic, so we can figure out what fits.
Here's a Vermont link:
http://www.chinariders.net/showthread.php?t=12368