In a nutshell, at the moment, there is no easy way to do that but some have tried it to varying degrees. Apparently there may be a very expensive way to mod the unit by sending it to someone for a mod which costa a small fortune. Others probably will come out with a reasonably priced aftermarket adaptor.
BTW, the NAV REC unit used in DC products this year is by Alpine and is one of the better units I have seen.
From what Ive read here there is a company named in one of the threads that can hook up a audio video in to the stock nav unit. This is an expensive custom job though I believe someone said $600.00 Search the forum a little closer and youll see what I mean.