Am I missing a piece/part on rear window?
 

See attached pics.
First is center of upper trim piece of rear window.
Second is rear window, center trim section, lower.

It looks like there should be a second plastic piece covering the gasket?

Any help appreciated.

Upper:
http://forums.pelicanparts.com/uploa...1316346524.jpg

Lower:
http://forums.pelicanparts.com/uploa...1316346545.jpg

nope, it looks short....that piece at the top and bottom should slide over and cover the gap

They aren't plastic... metal.

Carefully slide them to the left... don't scratch; use a flat screwdriver covered w/ cloth.

Both replies are correct. However, it looks like your trim is too far apart, especially in the top photo, for the sliding piece to cover the gap. There should be a gap as the trim actually helps seat the seal into the corners but usually the gap less than 1/2 inch. I don't know the year/model of your car and you might have the wrong trim for the year of the car, though the dimensions were the same for many years. If you have the correct trim size, you might try to massage the trim away from the corners a bit to close the gap enough for it to be covered. Be very careful, however, the trim is very soft and easily dented or bent. If that fails, you can either leave it, or (ugh!) remove/replace the window and seal.

Also, keep in mind, if the black trim comes out of the rubber, you will likely have to pull the windshield to get it to sit flush again.

I have the same issue with mine. I ordered the replacement sliding piece but the gap is approx 2.5 inches and cover is only 1.5. The passenger side was starting to lift up a little so I used some auto grade double sided sticky tape to hold it down. Can't really notice it's missing, but I know...