Did you try putting a banana peal in the flux capacitor?  The potassium is critical for lubricating the quantum chronologer.  In the post 2407 models the shift linkage was removed and replace with servo-electrical controls. The chronologer's C₆H₇KO₂ sensor will prevent the transmission from going into reverse if it detects low levels.  It's a safety feature, and provides much more accuracy for reverse travel.  One time I took an old model to watch Mt. Vesuvius erupt, and I had to wait forty minutes, so annoying.  I can't believe that Game Stop didn't recall those old models.