^ also make sure that the truck doesn't move -- you can block it with a car from the front to avoid it moving -- then you can take your time firing at the back. as said, only 2 shots and the doors will blast open. but if the driver (or passenger) guard get out to fire at you, don't return fire. you'll get 3 stars as normal upon firing at him.
It didn't move, but the guy hopped out, so I shot him and three stars.
So i will once again have to see if the trucks spawn up tonight.