BE CAREFUL!
At least from the wiring and switches it should be impossible to engage reverse, if the overdrive is switched on! If you do this, you will certainly kill your overdrive. The planetary gears in overdrives are intended for rotation in just one direction! If you use reverse with overdrive, the gears will stuck and that makes the judder feeling.
So normaly you should have electric switches on top of the box wich allow overdrive engagement only in 3. and 4.
Best wishes
Chris