Ok guys, I found out the trick to doing this, and soap and water are NOT it, not that I tried that. If you use soap and water on a water tight seal.....what do you think is going to happen??? The water's going to get TRAPPED in there and RUST your car. No thank you.

The RIGHT way to do this, as I found out, is to put the plastic piece in first, then use a rounded putty knife (ie, without sharp corners on the edges) and pry the rubber in from the inside and outside of the rubber. Then it went in real well.