autometer tach not working?? please help

i installed my tach today. when i turn over the igniton (before i start the car) the needle jumps around between 0-1000pm, so i know there is power to the tach.......i spliced and connected all teh correct wires...i think

green----brown with yellow stripe wire
black-----stereo ground/ black wire
red & white-----stereo pwr/ blue wire

any help you guys can give me woudl be appreciated