Resistor for a LED with high limits
Typical automotive switch. You already know the light could be always on, or only on when the switch gets pressed. If you switch the power and accessory pin, the light will change.



simulate this circuit &ndash; Schematic created using CircuitLab
The one on the left always has the light on. The one on the right only turns on the light when the switch is &quot;On&quot;.
But what you want is to control the led from an Output, and the Button as an input. It's still simple. Kinda.



simulate this circuit
The one on the left is simpler. Connected to the 3.3V pin. Input without pull-up resistor enable, active high/1 for button pressed. Output High/1 for LED off, Low/0 for LED on. You must change the output for 16mA or less, and the light should only pull under 16mA, because that's the max for the RPI.
The one on the right is better. The RPI 3.3V rail is limited, and if the light is a lamp it may pull too much for the 3.3V rail OR for the GPIO (Use a ammeter/multimeter to measure it's current), or if it is an LED may be dim at 3.3V. This way you use the 5V rail for a brighter light, and a transistor to switch the light with normal High/1 for On, Low/0 for Off. The two resistors on the input are a simple voltage divider. When pressed, the input will go high like before.
In any case use a multimeter to find the pinout and light current.