Maybe for rear HVAC?HVAC control function.
No C6 here, just the infamous E40D.@LowTech It is a vacuum solenoid, but I am not sure for what. Check the vacuum lines going to your transmission if you have a C6 and make sure those are good and that they didn't route through that unit. I'd guess some sort of cruise control or HVAC control function.
That is very possible. It did have a rear heater box that we pulled out and I can almost guarantee that it had cutoff valve inline. The control for it would have been in the overhead, so definitely not a mechanical one.Maybe for rear HVAC?