Hmmm... Ignition light...
LEDs arent just a simple matter of pluging them in... they need a dropping resistor so they dont get blown by the 12v. 50ohm resistor should be enough (LEDs draw 250mA dont they? I cant remember....)
Nonetheless, its still not as simple as that...
You can have it running off 12V mains supply... meaning its running 24/7... but i dont think you want that.
Um... running off the door switches are also, pretty useless... cause that would mean the main light is on. Which would make it pretty redundant.
Adding a switch is the only quick solution.
You COULD be hardcore and add a delay circut as seen in many newer cars, that gradually Dim the internal light after the door has been closed. I think i saw them sold at Jaycar not to long ago.
But, if u just want an led... wire in a resistor, K goes to positive, A goes to negative, bam your done.