I went to a fabric store and got cloth that would make good-looking carpet for my Jeep Cherokee. I had to go to about four or five different fabric stores before I finally found some cloth that "looked right" AND was the color I wanted without being too thick, so be prepared to break out the Yellow Pages, jot down some addresses and do a bit of driving around. I even brought the interior tub with me to the fabric store for testing various textures and thicknesses. For cutting the cloth to size, you'll need a good sharp small pair of scissors, so buy some at the fabric store if you don't already have them. Through careful measuring plus a bit of trial and error, I first made a pattern of the interior tub on graph paper, cut out various holes for consoles, seat fittings and the like, then after fine-tuning, transferred this pattern to a thin cardboard template (more durable then the graph paper). After further fine-tuning, I placed a piece of cloth on a flat beer box placed bottom-side up, securely pinned the cardboard template by poking LOTS of pins through the cardboard, cloth and beer box, then carefully used both the small scissors and an Xacto knife to cut out the "carpet." Again, this might take more than one attempt. After a futile attempt at using double-stick tape to attach the carpet (it doesn't work at all for permanently attaching the carpet but for doing it temporarily for test-fitting it works good), I removed the tape and carefully used tiny drops of super glue. Use the tiniest drops possible--too much will soak through the cloth. Do a small section of carpet at a time and make sure it's nice and flat.
I've also been to model car shows where the contestants used a product called "Fuzzy Fur" to replicate carpet. Used correctly, this looks even better than my cloth carpet (it might even be less work but I myself have never tried Fuzzy Fur as I'd never heard of the product until after my Cherokee was carpeted). Unfortunately, Fuzzy Fur is no longer being produced but with a little luck, you might find some in a good hobby shop or a model show. I think it's Detail Master or some such that also offers a similar product to Fuzzy Fur but I always thought the "carpet" made with this product looked a little too thick--maybe it's just the modelers that use it. There might be some other products I don't know about or other ways of adding carpet to model car interiors. Any takers?