Thank you very much for those suggestions 78F350. The power lines for the c/o valve in fact have their own hole poked through the harness firewall grommet and then head straight to the relay box on the far left of the trunk. So it seems that you were right about this being for a previous valved muffler or some other accessory that was removed (or installation started but never finished). I double checked that the c/o valve defaulted to closed by removing one end out of the Dorman sleeve and diving ~50 miles ... smooth running and no codes. And good to know that a vacuum line has already been plumbed-in if I ever decide to install PSE. Thanks again!
