So if the resistance is changing, I would believe the current is changing, since the 12V is likely to stay constant, right? Thus its a matter of sensing the current, not a voltage. Easiest way to do that generally is to put a small resistor in your signal line. This should be a couple orders of magnitude smaller than your button resistors (if they're already in the 10's of ohms range, this method may not work). Take both sides of the resistor and sense the voltage with a comparator. The voltage difference is proportional to the current, which is inversely proportional to the resistance applied by the buttons. You can set thresholds on the comparators to trip for each button. I dont know what your board is, but it may even be able to set comparators between inputs.