The plastic cover he's referring to is the body color panel directly in front of the door. It'd held on with a few screws, and covers the hinge bolts. The cover is missing and you can see the two forward most hinge bolts(there are four) in this pic:
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Your door holding issue is likely a broken spring, it's a fairly common problem. Snake Oyl sells a
kit to repair it. I'm cheap, and for the cost of the kit, I just let my door swing shut.
It's helpful to have two people when adjusting the door, and make small adjustments. If you're by yourself, use a floor jack with some padding to support the door while you loosen the hinge. As stated above trace around the hinge on the body so you can put it back to it's original position if you need to. Put some masking tape and a towel on the sill cover, and tape the edges of the door so you don't accidentally scratch the paint. It's not hard, but can be tedious and frustrating if it's your first time doing it.