I have been doing some research on this (the RX is apart in the garage)... I have a 2004 RX 330 with Navigation and Mark Levinson. This is what I found out:
The place to add Audio Input is from the Rear Entertainment harness. This intercepts the DVD audio going to the head unit for playing DVDs, it DOES NOT interfere with the headsets in the rearseat. From there you have R+, R-, L+, L- and ground. I DO BELIEVE THIS IS BALANCED AUDIO. I purchased a Balanced Line Transmitter (Zapco SLB-TP) to drive from Line Level input to a Balanced Output. I haven't tried it.
I also will add a switch to this located to switch between the factory DVD unit and my MP3 player. The only issue I found is that I can only select AUX on the head unit when I have a DVD present in the player. I am able to intercept the audio signal and play it in the whole car.
Does anyone have any better places to make this modification? I am open to suggestions.
Regards,
Nav