A jack, some wire, and probably just some cuts and patches to one of the circuit boards inside. ...
" I also add in the aux input , it need to play the cd for aux work, the way it work is when play cd and plug in the aux, it will swap the cd audio, the aux sound is the same as your CD music "
I guess you would have to keep CDs in there playing while you use the aux input. Probably won't work once the changer jams.
I put aux jacks in a 97 Taurus with cassette and an 03 with the CD player. The Tauruses were easy, as the amp+tuner was in the trunk (factory). The dash piece was the CD/tape player, radio controls, and clock. You just solder in an aux jack to the player's preamp signal wires to the wire harness in the back of the radio. My iPod cuts the cassette's volume by about 75% when turned on, so I rarely hear it. For the other car, I burned a CD with a blank track using Audacity. The cassette deck seeks forward when it encounters silence, so I live with Jimmy Roselli occasionally singing through
But as for the eBay decks... I don't know. The question is do the CDs play normally when the iPod is disconnected? Do they play at reduced volume when the iPod is plugged in? Or do they never play?
If they play at reduced volume when connected: probably a simple tap into the preamp signal.
If they never play: probably cut the preamp wires/PCB traces, attached Aux wires
Play normal when iPod disconnected, nothing when connected: splice aux jack in with some kind of interrupter.
Ahhhhhh...... These type of short cuts usually don't last long or ever work correctly.
Admittedly, my Taurus' hackjack plays the cassette's sound through (though at a reduced volume), but it has lasted about 3 years so far. I originally ran the jack to the ash tray, then later put it in the arm rest.
My mother's Taurus worked fine with the blank CD for the ~1.5 years she had it before trading it in. The only problems I could see happening were the jack wearing out (cheap eBay pieces) or the CD player wearing out due to more frequent use.
However, that's not to say other systems are just as reliable, installed as well, or only use basic components. I'm just throwing my anecdotal experience out there haha