So i know this part is rice, but I have always wanted one since back in the 80's and now i got one cheap. So heres the deal. I have a 97 JDM ITR. I installed a tach on the dash, ran the wires as follows.

Red power - connected to yellow power wire coming out of gauges.
yellow lights wire - connected to yellow power wire coming out of gauges.
black ground - connected to black ground coming out of gauges.
green tach sense - connected to blue wire coming out of gauges.

So all wires are connected to the far right plug where the tach is on stock gauges. The stock gauges still work perfectly, but the new tach stays at redline the whole time it's on. it jumps around when i hit the brakes, and when i have the blinker on it jumps with the blinker. I am wondering if the ground wire is not working? any help is appreciated.