I did this several weeks ago, but I've been rather busy and couldn't get photos up until now. They are in my gallery, and you can see the results:
http://www.318ti.org/gallery/showgallery.php?cat=816 The stain in the backseat has been there since 2003, the result of takeout leaking through the bag. Since my ti is a daily driver, it's been on ski trips, beach trips, camping trips, pretty much anywhere it can get gross. On occasion, I've left the windows open parking outside. The pictures show about 10 years of accumulated filth, which came out with the water (the first picture).
I really wish I had done this sooner. In fact, I plan on doing it again since I ran out of chemicals when I'd finished the first pass on the front seats (the rear really needed a lot of work). I'm very impressed with how well this worked, and how inexpensive it was.
You'll need to rent the Rug Doctor and the upholstery wand, and purchase the upholstery cleaner and anti-foam. Everything together was about $35. I'd actually suggest getting twice the amount of chemicals, as you'll probably want to go over areas more than once.
I did not experience any discoloration using the chemicals, but then again I don't think anywhere in my car was the original color anymore. YMMV.
A few tips:
1. Allocate an entire day for this. The more time you take doing this, the better results you'll end up with. The lower part of the rear seats alone took a few hours. You'll also probably need this to dry overnight as well. I cracked opened the trunk, shut off the light (pull on the switch), and opened the windows about 1/3 with a box fan inside. Feel around the footwells to make sure air is getting down there too, and adjust the fan as necessary.
2. Soak areas with the solution enough for the color of the fabric to darken. Slowly pull the vacuum wand over the upholstery until you don't get bubbles rising out of the fabric. Overlap at least 1/2" with each pass, as the solution "bleeds" by capillary action. I did one pass vertically, one pass horizontally.
3. I then soaked the areas again, with a vertical pass and a horizontal pass. I pulled as much solution out of the fabric as possible after that.
4. Make sure your wand is getting good suction. I used duct tape on the sides to ensure all the vacuum was concentrated on the open end of the wand. Some areas are difficult to get, as the wand doesn't completely cover it. Tilt the wand, or use a finger covering the open part of the suction.
5. Anti-foam works best when the foam coming through the hose drops onto water. My garage floor is slanted, so I had to move the machine around.
6. Earplugs are a MUST. These machines are loud.
7. Keep towels around at the bottom of where you're working. The solution will often foam up and drip.
8. There are some parts that are inaccessible by both the spray nozzle and the vacuum part of the wand; namely, the edges/corners of the footwells and around the accelerator pedal. On the next run, I plan on using a spray bottle, narrow handbrush and a shop-vac with a micro-cleaning attachment kit.
I give my experience with this machine and its effectiveness a big thumbs up. Price was low, but easily worth more.
I'll field any questions you might have.
Using a Rug Doctor to clean your upholstery 
Linear Mode
