do this-

-look at brake pedal
-find where switch hits the pedal (to turn off brake lights when its not being pressed down)
-look where the switch rod hits the pedal assembly...theres probably a hole there where the rubber bushing thing used to be.

-scout junkyards, head to nissan, or discount or homedepot to find something to fill it.

99% thats usually the problem. older cars the little rubber grommet gets all dried out and old and cracks and falls out. hell u might even find it on ur floor somewhere.