Try the Car Behavior's "On Bounce" condition instead of On Collision.http://dl.dropbox.com/u/529356/car%20crash.cap
On Collision checks for overlap between two sprites. It's basically the same as this:
+ Car is overlapping Wall
+ Trigger Once
-> Do stuff
Since the car is bouncing away from the wall before it's overlapping then you will never get On Collision to trigger.
On Collision has been modified for other behaviors in the past though. For instance, you can use On Collision with Physics objects to detect collision even though they don't overlap either. You might want to make a feature request to have Car recognize On Collision in the same way, if it's important to you somehow. Otherwise just use On Bounce.