Ok, I got the Nav head unit today. It plugs RIGHT in where the 500 goes. It turned on and works good, except there is no GPS signal because there is no GPS antenna in my car. Everything else DOES work just fine with the head unit though. I will have to say one thing I did notice is that the sound quality is not as good as the base Shaker 500.
The Nav system is made by Pioneer, and I have searched online and found that the newer/newest Pioneer NAV systems use what looks to be the exact same plug in antenna that I need. So tomorrow is my day off, and I will let you guys know if I find an antenna locally and let you know how it works out. If it all works out, I will go ahead and do a write up on how to/what you need to convert your Shaker 500 to a factory Ford Nav unit.