Detecting 12 volts DC

I have an application for a arduino that needs to sense the presence of 12 Volts DC on a specialized commercial vehicle. This vehicle has turn signals, brake lights, warning lights, backup lights, etc that I want to log when they are activated. (don't worry about the logging, that is a separate system and is out of scope.) It uses standard 12 volt LED lights that you would typically find on semi trucks. The manufacturer has said any modifications to the wiring will void the warranty.

So, I need a sensor that can tell me when the light is on. My needs are simple yes the light is on, or no the light is off. (A photodiode isn't an option because drilling holes in the body for the wiring isn't going to go over too well for a number of reasons.) I've been looking at Hall Effect Sensor switches, but I'm not seeing any talk about them working in my application. I've also seen some conversation about reed switches, but again nothing stating that it would work.

I'm relatively new to this world, so any suggestions about what and where to purchase this sort of electrical gadgetry would be appreciated greatly.

Reed switches are used to sense something that tilts.

Perhaps this might help: http://www.scienceshareware.com/how-to-measure-AC-DC-current-with-a-hall-effect-clamp-.htm

Thanks RW - have you seen anything that can be placed over existing wire? My goal is to place the sensor in the same compartment that houses all of the fuses and there's no way short of cutting wires to use those sensors.

I looked up the specs on a typical light and they draw ~290 milliamps @ 12VDC