Why does screen go dark when tilting too far back or forward?

I have a MacBook (Black) model about 3 1/2 years old.

The problem that recently started is related to the angle of the screen when open. The brightness and the light adjustments all work normaly, except when pushed back past 100 (est) degrees as well as forward past 70 (est.) degrees the screen goes to the darkest setting just barley showing its content. The screen operates with its norman brightness when in upright position within those 2 angles.

Genius bar people said it maybe caused by pinched wire, but couldn't further explain unless they took apart to examine. ($200. ++).

I would like to fix this my self as I have been 'inside' before. I would like to find instructions for this if possible.

Bruce