i have a 1980 300sd the passenger front window will not go up w/o sticking.i have removed and checked both motor and regulator. the regulator looks like new,teeth match perfect.the problem is window goes up crookedly,the back of the glass goes up first and after a couple inches jams. i noticed the arm from the regulator only pushes up from the very back hence the pressure to go up is always on the very back of the slide.any suggestions anyone?