EDIT: Got it! A friend taught me.
It would be nice if you could post the solution, a hint, or a link when this happens. Otherwise this truck is useless, or worse, since it comes up in my search and even Google:
Also you can't attach entities except for track-train-controls, etc. So I'd be interested in how you solved this. You should be able to put the damage right on the door, or activate and deactivate a trigger_hurt.