You're a T6 so you should have start/stop and a smart alternator?
In which case one of those options wires should be connected - you need the red option wire connected to a +12v source to configure the CTEK to a smart alternator profile. Without that the CTEK will only be engaging in the brief moments the Alternator is fast charging your starter so you'll be getting a poor charge.
Don't connect the black wire to ground (which seems common-ish) as that will select AGM profile and slightly overcharge a normal lead acid battery (which I'm assuming yours is)