NorthWestCuda,
I have a 71 Challenger R/T and a 73 Cuda and I've installed window felts on both of them. The inner felt that you're having trouble with should slide over the top edge of the door panel. A correctly formed felt does not need any glue to hold it in place. It is an interference fit where the channel in the window felt is smaller than the thickness of top lip of the door panel.
Here are some things that you can check:
1) Take a look at the original felt that you removed. If it has a slight curve in it anywhere, you may need to slightly bend the channel before you install it.
2) Look at the top edge of the door panel (where the window felt goes). If the door panel plastic is crumbling and not solid, your felt will not grip the door panel.
3) Lightly position the window felt on the door panel and see if there is any resistance when you push it over the lip of the door panel. If it easily slips over the lip of the door panel, then the channel in the window is either too large or the lip on the door panel is too thin. Remember - you need an intereference fit for the window felt to grip.
Hope this helps. I know this should be simple task, but sometimes turns into something more.