Spent the morning dyeing the carpets back to black. I used Autosmart Midnight carpet dye and a sprayer. Tried a small section as a test and was surprised how quickly the carpet returned to black. I then set upon all the sections. Used about 3.5 litres of the 5litre can.
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.
The full carpet set is now in the basement to fully dry, although some smaller pieces had dried to touch after an hour, I'm leaving it for at least a week to fully dry up and inspect outside how the dye has taken. However just looking at it all now I'm really pleased with the outcome and can't wait to get it all back in the car. For the £20 for 5 litres I paid its really worth the effort. Due to the potent smell I would recommend doing it with the carpet out of the car and allowed to fully cure before returning and driving.
PS I now have for sale a full 1984 924/944 black carpet set which I bought to replace my faded set. But as I've dyed my current set I don't need it anymore. The set came from a 35,000 mile fully garaged one owner 924 which was unfortunately written off. Full black, unfaded and near perfect except for the passenger rear boot section which had about one inch torn away in the accident, hardly noticeable.