detmar tachometer help

[dragnracer1]

i recently purchased a detmar 6k rpm tach with hour meter and i was wondering a couple of things
1 how do i change the bulb for the back light
2 there are three poles on the back one is marked ground one is marked sig and one in the center is not marked is this 12v+ switched or is this one not used
3 next to the signal pole is a switch marked 1 2 3 4 im assuming this is the selector for the motor i have a 1988 rude 88spl 2 stroke what possition should i set the dial too so that it reads accuratley

thanks in advance for any help you can give me