I had an aftermarket stereo installed

The factory dvd players communicate with the radios over the "data buss". That's the same wires your home computer uses for all the chips to talk back and forth. Aftermarket radios don't use a data buss so there is nothing to turn the dvd player on.

I don't do anything with aftermarket stuff. I repair only original equipment radios, and almost nothing but Chrysler products because of their high quality. For that reason I wouldn't be familiar with any type of adapter box to hook to your dvd player. Doesn't mean there isn't something available; just that it's not my area of expertise. You can try looking for something at Crutchfield or at a store like Best Buy.