Define hit area?

hei,

how can i achieve the following: see swf-file

the grey rectangle should start fading in when i reach the red square (hit area), not already when you cross the edges of the grey square…

the “button” is a movieclip.

see also pdf:
first layer (layer 3) has the button-coding
second + third layer contain the motion-tweened rectangle/square.

thanks for helping :te: