The later Volkswagen Jettas come with headlights always on by default; this feature is called daylight running lights (DRL). Unfortunately, DRL lowers the lifetime of the headlights and lowers the gas mileage (just a wee bit). This hack will disable the DRL only and leave the rest of the headlight functions alone. (Click on the picture to get a larger version.)
Note: This info is replicated from another website with my own comments added.