So, for those trying this out, the circuit didn't work as pictured for me. I'm working on some improvements that should get it going. We'll see how it turns out. One good thing is that the voltage divider setup I created for the buttons worked exactly as expected (One note: The buttons show 71 kOhms when nothing is pushed). Pins 2 and 12 were the correct pins after all on the head unit connector (REM+ and REM-). I connected ground to REM-. Pin 2 is pink, pin 12 is violet.
My plan is to replace the 2.2 kOhm resistor for the wheel with a 5k multi-turn trimpot. That should give me the adjustability I need to "dial in" the wheel controls. Failing that, I'll replace the two resistors adjusting the current/voltage for the IC with appropriate trimpots to dial in the voltage. Part of the issue is that in the real world getting everything exactly perfect doesn't happen. My expected voltage was 7.71 volts, but it is actually 7.5. Also, the voltages on the voltage divider for the buttons are off. That results in the wrong relays being triggered, and multiple buttons being mapped to the same relay.
Hopefully in the next week or two I'll get this ugly prototype working. Once that's done, I'll update the schematic, give some more useful instructions, and I plan to design a proper printed circuit board so this can be easily replicated. I'm of the opinion now, though, that at least one trimpot will be required to be custom adjusted to your specific situation when you install, but if it can be kept to just one, it shouldn't be too hard to do.
Yes, for those who love microcontrollers, this would be WAY easier with an arduino. Like 100x easier. Maybe even easier than that. But then it would cost more and wouldn't be as fun as doing it the 100% analog way. Plus it would up the parts count to many times more than you see here, necessitating you to buy a pre-made arduino board rather than just having the entire thing custom.
BTW: I did a couple of basic tests on the head unit already and it does accept button "presses" as advertised on that site. So that's encouraging.
View my fuel log 2015 Mirage ES 1.2 manual: 41.6 mpg (US) ... 17.7 km/L ... 5.6 L/100 km ... 50.0 mpg (Imp)