How to weather plastic canvas and how to open a closed door?

The two bumps are most likely supposed to be the hinges.  I would remove the bumps first, then using a straight edge and score a line where the door seam is.  Next, bend and break the plastic along your score line.  Sand the newly seperated door/body pieces to get a sharp line.  Then add the hinges back to either the door or the body piece.

To make the tarp look like fabric with damage, thin the underside with a rotary tool so it is thin enough to poke a sharp hobby knife through it.  The knife will leave jagged edges like cut fabric.  To paint the fabric, use a brush and do a couple different coats brushing in opposite directions.  This will give it some fabric texture. Lastly, drybrush with a different color to give it some variation.  Another option is to rebuild the whole tarp using tissue paper and thinned PVA glue (Elmer's school glue) to give it the texture and look you want.

I like to use preshading on fabric...paint the canvas very dark base coat, and then layer the lighter color with the airbrush:

img.photobucket.com/.../FAMO887.jpg

Arty gave some good advice, but to be honest, unless there's a lot of detail on the other side, I'd just scratchbuild new doors and the rest of that part, It would be a breeze, and a lot nicer that what you'll probably end up with. Such a simple piece of engineering would make good practice and build confidence as well. :)