For the first part you don't need scripts.
Place your car and make the first waypoint a hold waypoint where you want the car to wait.
Now place a trigger. There are a few ways you can do this but one way is like this.
If you want one particular unit you need to group the trigger to that unit.
You can do that by selecting group (f2)
from the menu and dragging a pale blue line
to the unit of your choice.
The next thing to do is syncronize (f5)
the trigger to the hold waypoint
Drag a dark blue line
to the hold waypoint
Open up trigger again and where it says type
and that should be it.
Take a look as an arrest script, there are several about.