I don't understand why you would need such a thing. I think you could plug in the Road Relay to the VP44, plug the box into the connector on the Road Relay and plug the truck wiring harness into the box connector. Stacked. You might need to reverse the order, box first then Road Relay for it to read right. BUT, the Road Relay might not give any meaningful data with the box connected, just like the trucks' onboard computers. Once you start artificially altering things with a box or injectors, they don't read right as their calibration will be off.