I am sorry for a potentially dumb question
Can I add the ring to black and red ends without taking the whole black (ground) off?
Yes. Two possibilities
1. If the ring diameter fits over the bolt that tightens to connect the cable to the battery post then remove the nut on the cable bolt slip, the connector onto the bolt, replace and tighten nut.
2. If the hole on the connector is too small to fit over the post bolt then use a side cutter to cut a segment of the ring to split it. Cut enough that you can force the ring onto the bolt. Removing half woukd be too m much. You will have to spread it since the hole is a lready too small. Loosen the nut on the post and place the charger cable split ring between the nut and post. Tighten.
Of course you have to connect to the correct polarity. I hesitate to say the obvious but most of this is obvious.
Are you sure you don't want to just snake the ford cable through the driver window? It would be less stressful