Do not do this.
The first time I had a CD player ripped off out of my car, I had installed a blinking red LED on the dashboard to make it look like an aftermarket alarm. The theives ignored it.
Well, I don't know if they ignored it. I think it helped them make the decision of how to get into my car. Instead of breaking a window, they used a crowbar to pry into its hatchback, causing even more damage than a broken window would have caused. I guess they assumed that it was an aftermarket alarm and that I wouldn't have gone to the extra trouble of wiring a switch to the hatch back.
How else to prevent theft? Well, do you own an empeg? (If not, why are you on this BBS?) How about pulling it out of the car and taking it with you? I do this every time. I even cover up the hole
so it also *looks* like there's nothing to steal (in addition to there actually *being* nothing to steal).
If you are dead-set on using a blinking LED, you can't plug it straight into your car's 12v wiring system all by itself, you need the proper resistor. LEDs will be burned out by the full 12 volts, but a resistor will take care of it. I forget the value, perhaps someone here can calculate the proper value for you. I seem to recall I installed a hidden switch that would turn the thing on and off for me rather than doing anything fancy to make it turn on when I had the ignition turn off. If you wanted to make it turn on when the ignition turned off, a simple relay circuit wired to the ignition wire would work.
But don't do that.