There should be one below that, that one will override the onboard graphic card.
The onboard is on the vertical plate near all the other connections, below is an internal card which is horizontal, in this case the horizontal one is the one to connect to.
Oh, I have mine plugged into the lower one, the one that is the graphics card put in by the old owner.
Also when inside the machine check the graphic card, if recent card there should be power connectors to the graphic card itself, make sure they are connected (into both sockets if there) they should be on the far right of the card itself.
And the last time I checked it was connected.