I think
@675430VME 's setup doesn't have a valve in the muffler, and is designed to be fully open all the time. The ECU needs to "see" the valve is alive and well, so he just had the controller unit removed from the muffler assembly and moved to the side, connected electrically but not mechanically. Whatever the ECU tells it to do, it will do it, but will not affect the flow of exhaust.
While you search for a replacement valve controller, you may want to consider bolting on or welding some sort of retainer on the muffler valve to keep it fully open, then relocating the Milltek v1 unit and valve controller. The Milltek unit should reply "everything is fine" to the ECU regardless of how messed up the valve controller is.
Is there any way to redirect pressure buildup in the rear wheel wells to flow thru the muffler space and exit from the rear mesh? Will be messy in the rain, but just a thought...