Edit: There are tons of DIN receivers with monitors which should be able to accept the video output from a navigation system. But, of course, I have no idea what you're willing to spend, or what specific features you want in a nav system.

In fact, I've been thinking about this...

His problem is that he doesn't have a double-din space in his dash, so he can't have the empeg and a nav system...

But all the nav systems I've seen require a double-din space anyway: One for the generic screen and one for the nav computer. So even if he just dumped the empeg completely he'd still be kind of stuck. Right?