scared stiff opto or ?? for crate. WHERE IS IT?

on scared stiff, where is the switch that knows when the ball hits the
crate and makes the "boom" sound? if you actually roll the ball slow
with your fingers and come close to the crate but dont touch the crate,
it will activate it so i assume its an opto.. i can not find it on the
pf, where is it hidden at? sometimes mine activates when i hit the
crate, other times it does not..
so i need to find it so i can fix it:) BTW-im not speaking of the
jackpot switch for when the ball actually enters and goes inside the
crate. im speaking of the switch that actitaes when you are increasing
hits trying to achieve multiball when the led's on the crate increase.
thanks