As I mentioned, these aftermarket harnesses don't always match originals 100%. Check to see if that extra blue is just branching off the other blue with the multiple connections coming out of the left side harness. Test for continuity between the two. If it is, you may not even need to connect it to anything. But, you should cap it off so it doesn't short out to something inside the headlight bucket because it will have tail light/instrument light power flowing through it at times.
So now, I think I have some good news/bad news for you. Good news - I think I've figured out your tail light coming on with the key issue. Bad news - I think it's due to some bad and incorrect advice I gave you. What clued me into it was the fact that your tail light still comes on with the key but doesn't come on in the "Park" position. I think I told you to clip the wrong wire on the ignition switch plate. For reference, here's that pic of the plate again. I just assumed that how the wires were grouped together was how they were connected to one another when the key was turned on or moved to "Park". I'm thinking now I was wrong about that .....
By studying another pic I have of the other side of the above plate, the side with the contacts on it, and the white plastic part with contacts that makes the connections on it, I've deduced that in the "Park" position, the red and blue at the bottom aren't connected, the red and blue at the top are. The contact plates on the white plastic part have 3 little contact "buttons" on them arranged in a triangle. This is how they fall on the switch contact plate in the "Park" position .....
In the "On" position, this is how they fall. One of the little contact plates on the white part connects the blue to the red, the other connects the red to the brown .....
So, to verify this, you'll need to test your ignition switch. Unplug it and test for continuity between the red and blue in the "On" and "Park" positions. I think you'll find it in the "On" position but not in "Park". The "fix" will be to disconnect the blue from where it is now on the switch plate and re-connect it to the contact you clipped the jumper off of. Sorry about this. Like I said, I just assumed the contacts were enabled the way they were grouped together.
On the bright side, we're learning here. I think getting rid of the tail light always on feature, and adding a headlight on-off switch on a later model, are mods many would find worthwhile and would like to do. It's a good thing that we're finally sorting out the specifics of exactly how to do it.