Again the need for such devices is due to local management failing to manage or discipline the workforce. As several contributors have said you can fit the seatbelt to integrate with different truck systems, but they are all flawed. If you interlock with the starter, then once started, the belt can be removed, If you interlock with run, effective BUT what happens if the operator pops the belt latch when he is going along? possibly no power steering, power brakes, (after a few applications), and iof he gets into trouble, reattach the belt, restart and then move, interlock with the transmission, pop the belt when travelling and you are coasting, technically not in control of the truck, and all can be fooled by sitting on the belt!
Best idea is to leave the truck alone, have a flourescent belt rather than black so it can be easily seen as a previous contributor suggested, and have it switching a light on the overhead guard. The light is on when the belt is not latched and the truck has drive is enabled or the engine is running.