How to detect and measure a step change in resistance
If you use 3V3 as a starting point and add a 1k resistor in series with your buttons, you'll get 3V for no button pressed, 0.76V for up and 0.16V for down.

The "up" and "down" voltages are well in the "0" logic level voltage, while the "no button" is a "1" so your GPIO pin change interrupt will have no trouble detecting them.

Also the ADC should have no trouble telling the difference between "up" and "down".

Don't forget to add a few nF capacitor at the input to filter noise which an automobile produces in generous amounts...