Audio inclined people--- help

[ut_cougar]

i have a question regarding adding on to the stock CDR-220... I was thinking about installing a traffic pro for the stock look but it is outdated and they want $285 for very outdated maps. So...does anyone know if it is possible to keep my stock CDR-220 in my 2002 boxster S and somehow run a DVD/navigation to the stock speakers, etc. What i was thinking was installing a single din DVD/nav unit in the glovebox (if it fits) and opening the glovebox when I need to use the navigation (it would be a fold out screen). But I want to keep the stock head unit in the dash so I can run CD's in it and listen to the radio and keep the stock 4 channel amp as well, but when I want to use navi or DVD or TV i want that to be able to play through the same speakers...maybe running the aftermarket navi unit to the stock head-unit somehow? if anyone can enlighten me as to whether this is even possible and/or how it's possible that would be awesome.