Originally Posted by DIGItonium

Blue/White LEDs are very sensitive. Even in the off position, there's still a tiny bit of current flowing through the socket (probably a solid state switch from the BCM). Regular bulbs have higher resistance, so it won't light up at all.

Try rigging up a blue/white LED. Fully connect one lead, and use your fingers to "connect" the leads on the other. It'll glow dimly.

It shouldn't drain the battery.