I'm all sold out and have no plans to produce more, sorry.
I've made plans and required materials available here on the forum. I'd have it linked in my sig, but for whatever reason I cannot add links to my sig anymore.
As for implementing a "3 temp switch," I'm gonna need more information about how you plan for this to operate before I can tell you yes or no. Offhand, I would say no, or at least "not well," because the harness I devised replicates the factory wiring and operation, and therefore all heater functions are actually controlled by the BCM which tries to hold a set heat level by cycling power to the heaters on and off based on what I believe is a timer function.
Basically, if you wanted the harness as I designed it to allow multiple levels, you would have to somehow program that into the BCM and alter the factory switch telltales to signal which heat level is enabled (in other words, not realistically possible). Else, you could put the level switch in-line of the heater power wires but this would require you to first hit the factory heater button, then set your level with another switch, and would furthermore have the drawback of only receiving intermittent power due to the BCM timer function. This could definitely work, but I cannot guarantee how well it will work in practice. You would also need to find a location for these level switches.
If you were to use an eBay seat heater kit with multiple levels or similar, you'd be much better off not using my harness at all, using whatever harness it comes with to bypass the BCM entirely and just tap into a +12v lead in the passenger fusebox that is only hot when the ignition is on (this is important so that the heaters are not operating while the car is off, draining your battery. I cannot remember if the spot my harness taps into is ignition-hot or not, so verify that), and placing the control/level switch wherever you like. That's the easiest, and most trouble-free way to get multiple heat levels, but will require you to find a placement for the switches. I wanted to avoid this because I did not want to drill holes somewhere in my console or dash for the aftermarket switches and just wanted to replicate the factory operation as it was intended. It's up to you, of course.
Now, with the control modules in the Focus seats, I do not have direct hands-on testing experience with them so as far as I know there is NO way to get the built-in modules and switches to operate correctly with the Fiesta. I do not know if the seat modules communicate with the Focus BCM in any way or if they are purely independent and simply need power supplied to them. It might be possible, but it is not something that I have explored and I do not think I will ever have the chance to do so.