I am trying to add LED approach lighting to my new car. I found a cabin lighting circuit used to control the footwell lights, but it is a negative DC circuit that runs between 6.25 volts (on), 1.8 volts (off). I tried to use a 12v relay with a 5v trigger thinking the negative would be the trigger voltage, but the 1.8 volts is enough to keep the relay open and the lights on. Is there something that once the voltage drops below a certain rating will keep any current from passing? I know I am in over my head on this and may just have to have the dealer add the ground lighting kit to the vehicle, but they charge more than $550. 
So I thought I would give this a try and see what I can find out. Thank you all in advance. 