2001 Toyota Avalon Video

I've got a 2001 Toyota Avalon with the factory navigation system and tv tuner. The screen is built into the dash, remote from the actual head unit. The head unit controls the tv tuner and radio while a separate remote control that docks into the area near the shift lever controls the navigation system. Early on when I purchased the car I installed a line level converter so I could enjoy some good aftermarket sound. I currently have a great front stage and rear stage setup but am now looking to add a DVD/MP3 player. Due to the fact that I will more than likely need to keep the factory head unit I was hoping I could get some opinions on a good way to integrate a DVD/MP3 player. I want everything to play through my existing aftermarket amps but I'm not sure how to accomplish it. I also want to add a flip down monitor. So, the floor is open. Let me know what would be best.

Thanks,