I've got one layer with a wiggle(4,100) and I have pick whipped it to a brightness effect (brightness and contrast) on another layer. I am gathering it's not working because the values are different, % vs no.
How to I change it to get it to work AND change the subtly of the effect, meaning that when the wiggle goes to say 4% opacity, the brightness value is not so dark you cannot see the image?
That depends on what you aplyed the wiggle to. If you've aplyed to position, that has 2 values, you cannot just pick whip brightness to entire position, but only to one of its values.
If you want both X and Y pos. to affect the brightness you can do something like this:
val = thisComp.layer("wiggling layer").transform.position+thisComp.layer("wiggling layer").transform.position/2; //evridge of the position x,y values
linear(val,1100,1300,-30,100) //this changes the resoult values (1100 and 1300 were my resoult values) to range between -30 and 100 (bright enaugh)