DIY Aux Jack Install

[Dave]

I've tried and tried to search for relevant threads to this, and couldn't find any. I think I remember some things from before the threads were erased last fall...
Anyway, apologies if this has been asked and answered before.

The stock stereo HU in my Canadian-purchased sedan does not have the AUX button on it, but I'm wondering if there is any way to hook up an auxiliary cable to use with an mp3 player.

I thought I remembered an old discussion or two where people found that their cars came pre-wired for an aux jack, even if one wasn't installed. The people then found that when a jack was wired in place, pressing the DISC button (or something) twice caused the CD display to read "track 99", and audio worked through the jack.

I found this thread regarding the pre-wiring: http://www.yarisworld.com/forums/showthread.php?p=67053

Any insight from others' experience would be greatly appreciated, before I start taking things apart.

And, yes, I realize it would probably be easier to just buy a new deck with a jack built in, but I think after-market decks are generally horrendously ugly and I kinda like the look of the stock unit.