ok, heres what I gather:
you have a seperate monitor, dvd player and head unit and you want those hooked up, correct? You have an FM modulator, but you're not sure if you need it.
Heres what I'd do:
That Panasonic has an auxilary in, correct? If it does, I would hook the DVD player up, and run audio RCA's from it to the aux in on the head unit. Then I would run video to the monitor and you're good to go. No need for an FM modulator.
If your deck doesnt have an aux in, then you would wire constant and switched power, and ground to the FM modulator, run audio rca's to the modulator, plug the factory antenna into the modulator, and then plug the antenna cable coming out of the modulator into the antenna jack in the head unit. And of course run video to the monitor.