adding dvd to factory system

The only way I could think of that being possible is getting a passive speaker switcher, like one you would by at radio shack that you would mannualy have to switch speakers A (the back which would be to the radio) to speakers B (the back which would be to the dvd player).  You would have to find a way to rig it up so it would be nice but that's the only way I could see that working....Hope that gives you some ideas

I wrote a reply to this yesterday and thought it got sent but I guess not.

Anyways, I was gonna say if you have an aftermarket external 2 channel amplifier you can use it to power the rear speakers, and the only thing extra you'd need would be some sort of RCA type switch that would allow two different RCA sources to be hooked up to the amplifier's inputs at the same time.  When you want to listen to dvd on the back speakers just flip the switch to the corresponding setting and you'll get the sound coming from the dvd player's RCA audio outputs. 

The only trick is that for the regular radio signal to go to the RCA switch, you'll need to use a line output converter and tap in to the factory radio's rear speaker output wires, because your factory radio does not already have RCA outputs.  The RCA outputs on the line output converter will be hooked up to the other RCA input for the RCA switch.

You'll want to get a line output converter that is adjustable (most are) so that you can match the signal level (and thus the volume of the rear speakers) that goes to the 2-channel amplifier with the level of the factory radio.  This way when you're using the radio as the source for all of the speakers in the vehicle, the ones being powered by the aftermarket amplifier won't drown out the ones in front that are still being powered by the stock radio.

The dvd audio won't need a line output converter, but since the dvd player will almost definitely not have a volume control built in, you'll want some way to control the dvd audio's volume independently of the radio's volume.  For this you can get a line level control like the PAC LC-1, which will adjust the signal level of the  audio  signal being output from the dvd player (and thus it would function just like a volume control for the dvd).  Also this would make it so that you can prevent the dvd audio from drowning out the radio volume in the front.

Technically you could just adjust the gain controls directly on the amplifier to achieve this, but since the amplifier would most likely be mounted somewhere not easily accessible, it just makes more sense to get the knob-type control.  Not to mention the gain controls on most amps are very tedious to adjust and some even require special tools.

just a note, although the PAC LC-1 says it "allows you to add subwoofer level controls to any head unit", that's not really applicable to your situation and the LC-1 DOES output a full range signal so it will work perfectly for your setup.