I just took the rear window out of my 450slc. It doesn't have clips, aside from the two outside that hold the metal trim in place, but it does have a defroster in it. The window came out very easily. Mine has an aluminum strip that runs around the outside edge of the window. It is not only for decoration but helps expand the rubber seal and keep the window in place. The strip is held together at the ends with 2 little sliding clips. These just slide back revealing the joint.
Take a wide screw driver or the likes and gently pry the edge up. I used a Wunder Bar, a wide slightly bent, pry bar that made the job very easy. It has a V in the end that fits on either side of the T that holds the trim into the seal. Don't go too fast or you will put a curve into the strip. The strip is very soft so I think that if you did bend it you could easily straighten it out.
Once the trim is gone I just cut the outside layer of the seal off and lifted out the window. BE CAREFUL if you go this way on a window with a defroster. Not only is there a wire that goes from the window into the trunk on the pass. side where the edge of the window goes from vert. to horz. (at least on my 450 slc) but the top and bottom edge of the window has a black plastic strip that covers where the defroster wires connect. If you damage this strip your defroster might not work again. Of course if you live in S Texas as I do, who gives a damn, we never have frost down here anyway.
I warn about the wire because if you take it to a glass shop that doesn't know this they often remove windows by sliding a wire around the the seal from the inside to the outside of the car. This might damage the connecting wire.
If you have ever thought about getting you head liner renewed you might want to so it now. My headliner, atleast in the back, wraps around onto the lip where the window fits. To do this job later means taking out the window again.
Also, when you take out the rear window, check for rust at the hole where the drfroster wire goes into the trunk. This is a prims area for rust. I just had a large rust bubble fixed there.
The cost. The new seal cost me about $100 and the glass shops wanted about $75 to $125 to take out the back glass and replace it when the body work was done. I took the window out so I reckon I can get it back in for $50 labor.