Did you take out the old DIN frame, or just take off the fold-down flap? If you take it out, what holds the unit in place?

Ok, just bought one of these for my 2003 4.4. So far, pretty easy to install. I now have everything cabled, and I'm ready to push the unit into place, however it does seem to be very tight. Am I doing something wrong, or do I just need to be brave enough and push the bottom end of the unit in tight?