I did this myself.
I have a 2005 Ford explorer Limited, and i have the RES and replaced the head unit with a aftermarket dvd system with aux inputs and outputs.
if you need help doing this, i can do my best to help you....i retained my factory RES, with video/audio out from the new head unit. and also run the RES independently from the head unit, and send audio from the RES to the wireless headphones.
everyone (shops and individuals and forums) said it was not possible in a 2005 ford explorer....but many hours of research, and time spent practicing...i did it...no problems..