Ed, thank you for pointing them to the Cover Extension. Where does one locate a part like they're recommending?
I just received this update from the Chevy service department:
We were able to track the signal loss to the r.h. tonneau actuator harness. (part # 89044555) - from me: but this part number is for one of the 8 cylinders (right stowage cover extension cylinder), not a harness
voltages are present but change when actuator moves,
but they are out of specified range at the extend switch.
Part is discontinued thru G.M.
We are trying to locate it.