And to the OP, if yours puts out varying resistance compared to buttons, not voltage, you need 1 or 2 resistors as explained below. These values are from a 7th generation Honda Civic and will not be the same on your car. You need a multimeter that can measure ohms.
If when no buttons are pressed you get a reading, that should be the smallest resistive reading. This is 100ohms. Now that is the smallest value. Because of the nature of the voltage divider, and we are trying to go from a maximum of 13.8v to a maximum of 5v and that 100ohms is the R1 in the voltage divider, that means the second resistor R2 (That you need to buy) has to be roughly half of that so 50ohms.
If you push no buttons and get a reading, you only need 1 resistor as is the case with the 7th gen Honda Civic. If you dont get a reading, or the reading is low (like less than 100ohms) attach a resistor between the R1 and where the FB connects like the last image #3.
So when you have no buttons being pressed, you will get 4.597v registered on the Fusion Brain. Now there are quite a few buttons, but the highest value button is 3.7Kohms, or 3700 ohms. So now R1 is 3700 + 100 (the way the civic is designed. The 3700 is from a schematic. If measuring in the car, you will only see 3800, because the initial 100 is already added to that measurement). R2 is still 50ohms. So now you have this:
So when that 3700ohm button is pressed, the FB will measure 0.179 volts.
Now these voltages and ohm values are approximate. When you buy a resistor, they are labeled with tolerances. Usually 10%, 5%, 1%, or 0.1%. The higher the tolerance, the cheaper the part.
What that means is if you buy a 100 ohm resistor with a 5% tolerance, what you are buying is guarenteed to be between 95ohms and 105ohms. So any value of 95ohms, 96ohms, 97ohms, 98ohms, 99ohms, 100ohms, 101ohms, 102ohms, 103ohms, 104ohms, and 105ohms is acceptable (and of course there is the infinate number of decimal numbers inbetween like 100.4543ohms and so on).
If you buy a 100ohm resistor with a 0.1% tolerance, you are guarenteed to have a 99.99ohm resistor to a 100.01ohm resistor.
And if you cascade 2 resistors of 5% tolerance, you will be at worst off by 10% in your calculations if each were off by the maximum of 5% and so on.
But the beauty is the value wont change once you buy it. It will always be wrong by x% where x is between 0% (dead on from the factory, exactly as described), and Tolerance% (the worst they are allowed to sell at that tolerance).
So do the voltage divider calculation with your exact values as measured by a multimeter, and that will be what you use.