ok so you basically want the LEDs to turn on for 30 seconds after you've turned the car off correct?
That shouldn't be too difficult.
You can use the always fun, easy to find and incredibly cheap 555 timer and then wire it up in "Monostable" mode
for more information you can look here: http://www.kpsec.freeuk.com/555timer.htm
you can use the ACC line as your trigger and then the output can go straight to your LEDs...
Depending on how robust the specific 555 chip is the voltage supplied by your cars battery might be too much for it, in which case you might want to consider some small bit of additional circuitry such as a transformer or power regulator to absorb any spikes and dips the electrical system might experience.
You can get a 555 timer and probably everything else you need at your local radio shack for a couple bucks... (you can get it for pennies online).
Feel free to shoot me a PM if you have any other questions, I do electronics stuff all the time. the 555 is a great beginners tool for electronics, nearly every class on electronics will have you using a 555 and an LED in various configurations, even that link I posted above is from a highschool electronics club![]()