Dissolve 1/2 cup of Borax in 2 cups of warm water. Sponge the mixture on the carpet, let it sit for a half hour, and then rinse well. Let dry and then vacuum.
To put the borax to the test, I found a dirty spot in our van that I had previously tried to remove with carpet cleaner with no luck. I have no idea what this stain is from since we bought the van used, with the stain. For your visual enjoyment, here it is:
I mixed the borax in warm water as directed. The borax is gritty and doesn't totally dissolve in the water. Rather than sponging it on, I used a scrub brush to work the borax into the stain. I waited more than 30 minutes, more like 3 hours before I rinsed it because that's when I got around to it. :) I have a Bissel Powersteamer which I used to rinse the area. Because borax is gritty, I don't know if I would recommend rinsing with a carpet cleaner because I'm not sure how good all that grit is for it, but I cleaned mine out pretty well. I was thinking that instead of a carpet cleaner, a wet/dry vac would probably be a good option for the rinsing. Lots and lots of dirty water kept coming up during the rinsing. After a half hour of rinsing, I stopped, even though more dirt was still coming up--I just had to move on with life. After it dried, I vacuumed as directed. Here is the final result:
As you can see, it's still a little dingy, but there is definitely an improvement, and if I'd have had oodles of time, I'd have kept rinsing until the water came up clean. But, it's just an old car. Some things are more important.
Borax does a great job at releasing stubborn stains. I have since used it in dingy areas in the hallway in my house and it has made the carpet like new. The rinsing again takes the longest because you just have to keep rinsing it until the water comes up clean, but it works!