How to build a blinking 12V indicator LED light
This is your garden-variety 555 astable set for about 1/2 second on and 1/2 second off, driving a transistor switch which turns the lamp on and off.



LAMP DRIVER DETAILS:

Here are links to the 555 and to the PN2222 data sheets, and the power budget for the PN2222 follows.

I wired up the transistor with a 24 ohm load instead of a lamp, and it's been running at 1 Hz, with a forced beta of 10 for a while now, and its case temp is at about 31C, so data sheet worst case spec's aren't even close. 

I'll let it run for a few days just to see what happens, but I don't expect any surprises.



A MORE SENSIBLE APPROACH:

Although about five times more expensive than a PN2222, an IRF7201 takes all of the guesswork about which 2222 is better than which out of the equation, and is a bulletproof substitute in this application: