If you have your headlamp wire connected to your illumination wire and those two wires wired to the switch then when the headlamps are on you are going to get a straight 12V. With the head lamps off you will be drawing only from the illumination wire which could be below 12V. Thats my guess anyway.

My hydroponics experiment

You ever notice how hard it is to lip read cartoon characters?
"Crazy is what the sane call Delta Flyer"