I have similar problems trying to install it in my 3100. I finally got it in there using a piece of wood and a hammer and also just a lot of hand pressure to get it going into the corners. I found that hanging out the door and pulling it into the corners is easier for me than trying to push it in. Once you get it going keep pressure and inch your way around putting a little more in at a time. It also helps if you have an assistant to try to hold it in place in the corner while you work down the line and into the next corner.

Hopefully you start at the right place and it makes it all the way around.

Mine ended up being about 3-4 inches too short. Not sure if that was a result of me triming it 20 years ago or that it was just short when I got it. But now it is too short and I have a gap in the back under the dome light (where I chose to have the ends meet.) Not sure if that is the correct spot but I thought I would see it less back there then in the front.