[tp550i]

The exhaust valve is controlled by vacuum. Its activated at an electric valve located upstream of the muffler. I never located the electric valve so I'm not sure if its located behind a trunk panel or under the car.

next time I have the car on the lift I'll try to trace the vacuum line.

It would be interesting to disconnect the electric valve fuse or disconnect the wiring harness to see if the valve defaults to the open or closed position. Of course this could set a CEL as well...