Sorry to bring up this old thread but it has helped me to solve the aux-in problem of my head unit. So thank you guys for your post.

I just followed the steps stated in the 1st post but since I can't find any available ai-net male plug, I have to solder some wires directly to the circuit board of the unit on pins 4, 5 & 6 of the ai-net port.
My unit is an old MDA-W700J. If someone has a similar unit with a missing manual, just follow these steps:
To access the unit's special functions including the "MUTE ON/OFF" feature, you have to press and hold the power button for several seconds until then you will be taken to the hidden menus. Here's a pic...
While inside this menu, the MONO button also functions as the MUTE toggle switch. It must be set to MUTE OFF...

Just press the power button once to return to the main menu.
It is also very important to locate the pink/black wire in the harness plug...
Then you have to install a switch in between that wire and the cars body ground. This is just a temporary switch which I used to see if the aux function will be activated...
By now, you will be able to toggle between the main menu and the AUXILIARY mode if you have properlly installed the switch...
If you want to connect multiple audio devices such as PSP, iPod, or external DVD players, you can also use a multi-port RCA hub for your own convenience...
Well that's about it guys. Good luck and happy hacking.
