A quick addition: I used a 430ohm resistor with a standard white led from Frys, and a momentary spst pushbutton switch.

I have a '96 850 and I tried to read the odometer value with no luck. The socket under the coin holder is labeled OBD-II. I used pin 16 for power, pin 5 for ground and pin 7 for data. Just plugging in the code reader caused the led to come on (with the ignition off and also in the accessory position...