What's that? Do I see a Z logo in the sky?? Haha!! I don't have a lot of time right at this instant, but I'll try to add some quick value where I can.
First, for reading the wiring diagram... The "O" means "makes a connection", and the line between "O's" shows you where that connection is made.
So for example with regards to the headlights... If you look down the OFF column, there are no "O's" because there are no connections. Then when you rotate the switch to the 1 position, there are "O's" in row's 4 and 5 which are connected by a line. This means that "In position 1, wire 4 gets connected to wire 5".
Progressing to position 2, "Wire 4 is still connected to wire 5, and in addition to that, wire 6 gets connected to wire 7."
In English: "When in position 1, your parking lights are on, and when in position 2, your parking lights AND your headlights are on". That's how that diagram works.
Second, that harness (with some caveats) is truly plug and play. It does not attach to anything under the dash, nor does it plug into any proprietary or unusual connector used just on the 280's. It makes connection to the three prong connector right on the back of the headlight. The intention is that you pull the three prong plug right off the back of the light and connect in that harness instead.
Now for the caveats... First caveat, as you discovered, is that the length of wires between the two headlights need to be long enough to reach from one bucket to the other. If this thing was designed for a go-cart, that may not be the case.
The second caveat is that you need to be able to actually physically get the wires into place, and here's where the Z's are a little difficult because of the way the headlights are physically mounted in the buckets. Problem as zKars alluded to is that the hole in the back of the headlight buckets isn't large enough to pass any of the connectors through.
Your choices are to make the hole in the back of the buckets larger, or to take the contacts out of the plastic connector shells and pass them through the bucket and then snap them back into the connector shells. Not too difficult, and if I was going to use this type of harness, that's probably what I would do.
Hope some of that helps, and if not, I'll mess things up even more when I get more time!