Sorry. I doubt anyone has an easy answer. It may be as simple as removing and replacing the vent. Then it could be a wiring fault or the controller or??? While it's a pure guess I'd think that something like that would be an LED. No bulb to burn out to give a simple and cheap solution.
You could try looking in PET. The Porsxhe parts catalog. There used to be a pdf copy in the site file section for the 997C. Probably still there. It would give you an idea if it was a bulb or not and maybe an idea how it goes together from the illustration.
I've concluded, with mild certainty, that there is no connection to the vent and it probably left the factory without this wire connected. There is a location for a 2-wire connection, but no wire leading to the vent (unlike the left center vent).
And now that my frunk trunk actuator gremlin has returned, I'm abandoning this project and moving to remove the front bumper. Many swear words were uttered. I disconnected the wire to the left center vent so at least the vents look the same (the outer left and right vent lights are functioning).