Anybody here without Navigation used the Navigation Dash trim to add a video screen? I'm wondering what size screens would work, and whether it would look "homemade"? Anyone?

I thought about it. Maybe this can help you:
http://www.trademotion.com/partlocator/index.cfm?action=getLocator&siteid=214027&chapter=ARF3625&appSectionid=2610&groupid=4805&subgroupid=2615&componentid=17366&make=25&model=Titan%20pickup&year=2004&graphicID=3625380&callout=16&catalogid=2

Find out the size of the factory screen and buy an LCD screen that will fit there, you might have to wire it and make brackets yourself, but I think it will work.
It's probably 7", since that's the basic size for dash screens

There's been a few people who have done this, most are using a 7" screen. Just out this thread.
http://clubtitan.org/postlite4554-llajumpvid.html

Will the Nav housing screen work off of the factory DVD player ?

I do not have the navigation but want to add the monitor up front.

What are the total cost involved with this mod ?

It's a little more work to get the factory DVD to work. Since the factory DVD player does not have RCA output, you'll need to find the video wire and tap some RCA cable to it.
http://clubarmada.com/showthread.php?t=39