Ron H said:
Is the reading of "1" not a measure of resistance, but an overrange indication?
Ron, I think it is an overrange indication, that's how it is shown on a couple of the meters that I have.
it seems they used three resistors (basically added one between the switch on the right and the output) and they seem to be about 100 ohms each, since the values are close to 100, 200, and 300 when switches are pressed.
if, for example, you used a 2k resistor between the output and +12v, you would get output voltages of 0.57v for the 100 ohm output, 1.09v for the 200 ohm output, and 1.57v for the 300 ohm output, and the maximum current draw of the resistor divider would be only about 6mA which is completely insignificant in a car's electrical system.
all you have to do is have a circuit that can detect the difference between +12 (no switches pressed), 0.57, 1.09, and 1.57 volts. As others have already stated, a comparator circuit would work, or you can use a bar-graph type driver chip, which is little more than a pre-made bunch of comparators set up for such a purpose. A very good reason to use the LM3914 type driver chip is that it can be operated in "dot" mode, where only ONE of the outputs will be active at one time. If you just used a few comparators with no logic circuitry on the outputs, the outputs for all 3 switches would appear to be on when you pushed the button that makes the output 1.57 volts, because that voltage is also greater than the thresholds for the other two switches.