I would never consider placing home-type fiberglass insulation in any vehicle I own. Each of those strands is like a mini tube, they love to catch and hold moisture. And not just from rain, even from simple condensation. If you must use that stuff, you should place it in a plastic bag, seal the bag, then stuff it in the cavity.
For filling cavities I try to use [expanding] closed cell foam. Better at blocking sound, and wont soak up and retain moisture. It also provides rigidiy to the surrounding panels once it sets up, unlike the pink stuff.
Never settle on just covering holes in your door with deadening mat. If you had a speaker box with a large hole in it, would you fix it by covering it with a layer or two of flexible mat? No, you'd screw a rigid piece of material to it for structural rigidity. If you use your doors for mids, large holes covered with mat will simply flap to the pressure changes. That's ghetto at best, performance altering and noise creating at worst. If you have substantial sized holes in your doors (more than a few inches across), use a rigid mat'l like wood or some form of rigid plastic to cover the holes. This will help strengthen the entire door and help reduce rattles even more. Once the hole is covered, then hit it with a layer of deadening mat.