Your 12V system in your car won't get up to 30V, 14.5 maybe. Put a 10uF cap on the output of your regulator and mount your .1uf cap as close to the µC power pin as possible. The 12V system should not vary that much that it would be noticable with your LEDs. But if it is, maybe you can power the transistor circuit from your 7805. The output will be stable regardless of what the battery is doing. You might want to consider using another 1N4007 in series. You could even use a 9V regulator to step the voltage down to the 5V reg. Your regulator will get a little warm with 8V dropped across it.