[ChrisW6ATV]

To me, standing upright is the best option, as that is the way the set is designed to be used. For the best security, face the screen forward, and if the truck has those loop anchor points at the four corners of the top edge of the bed (or if you can pick up a couple of them-cheap at auto parts stores), tie a rope or strap around the set from the left front anchor to the right front one. Ideally, you would check inside the cabinet first to be sure the chassis is held down by at least two of its original screws, too.

Second best position, if necessary, is to have the set face down, but then you need to have the chassis secured for sure, and you also need to add supports (2x4s maybe) under its corners or elsewhere to be sure it does not lean on the knobs or their shafts.