A quick tip from a friend of mine that runs a body shop on fabricating plastic vapor barriers for the doors.
Glues can have several problems. Spray adhesives can be tricky to apply to the door without getting the glue all over the place. Trying to re-glue after pulling back part of the barrier when repairing something inside the door can be problematic. Glues can dry out over time, leading to water getting to the door cards.
At the body shop they use 3M Strip Caulk to attach the plastic. Strip Caulk, for those of you not familiar with it, is caulk that comes in strips 1 foot long and 1/4" in diameter. It's a soft non-hardening caulking material that never drys out and can be easily shaped by your fingers. It is also great for sealing all types of seams and joints and can be found at any auto body supply store. If you want to speak the lingo, the auto body guys call it "dum-dum".
Todd
The first step is to cut the plastic to the correct size. Leave it a couple of inches too large - you can trim it down later.
Run the strip caulk around the door opening by butting one strip against the next, slightly overlapping them. Use your finger to press the overlapped sections together so there is no gap in between. The key is to get a continuous line with no gaps.
I like to run the line of caulk at the bottom of the door right at the edge of the metal, if possible. If the caulk is below the edge of the metal you end up with a "pocket" that can trap water.
If you're really a perfectionist you can do like I did and tape a flap to the inside of the plastic that will hang over that metal edge and deflect incoming water from getting to the lower line of caulk.
Once you have a continuous line of caulk around the opening, lay the plastic on top. Use a bit of finger pressure to adhere the plastic to the caulk. No need to squish it flat, just enough to ensure it is stuck. You can easily lift the plastic off the caulk and reattach it if needed.
Now you can use a sharp razor blade to trim the plastic down just outside the caulk line.