I dyed my 924 black carpets with Autosmart Midnight carpet dye with excellent results. Must be done outside the car, they will need to be well hoovered before refitting to remove the black dust, they are water and even petrol resilient......yes petrol was an inch deep in the footwell a few months after cleaning and dyeing, not amused!!
Here you can see the left side already sprayed up to the top of the transmission tunnel. The right side has a faded area which would be the rear passenger footwell. The right side front would be under the drivers seat and protected from the damaging UV.
A little closer. You can see the dye bleeding into the faded untreated area which would be the top of the transmission tunnel. The tunnel top was one of the most faded sections in the car.
Now fully sprayed
It seems the longer the dye is on the carpet the more the colour takes. The dye bleeds into the weave of the carpet the longer it sits. In the worst affected areas I also worked the dye into the carpet with gloved hands and recoated after 20 or so minutes of drying time and repeated as required.
Below is after one coat and already the faded area is almost as black as the protected and unfaded area to the right.