There is another post out there about someone installing the AUX Jack into their base fit. I copied the interesting part about wiring up the jack below. You should follow the link and read more.
http://www.sowbug.org/mt/archives/000256.html
QUOTE...
Eventually, I decided to take matters into my own hands and soldered ten RS-232 female headers (about $4 for 100 at Fry's) onto the ends of five wires from a cat-5 cable. I wrapped each in electrical tape to prevent shorts. Using some info I found on the web, I concluded that the following pins matched up (5-pin on the aux jack, 20-pin on the radio): 1-15, 2-5, 3-3, 4-13, 5-14. (For the person who buys my car in a few years and wants to know which wire is which, the colors going into the aux jack are 1-O/W, 2-W/G, 3-G/W, 4-W/B, 5-B/W, O=orange, G=green, W=white, B=blue, and "O/W" for example means "orange wire with white stripe.")
Next came the hard part. I pulled out the head unit far enough to reach in behind it and held a mirror in back. Then after approximately 1,000 attempts I successfully pushed all five headers onto the right five pins on the back of the unit. Same with the aux jack, but that one was easier because I could look directly at it. I plugged in my MP3 player and confirmed it all worked ...END QUOTE