Seats are often the first thing you see when getting in a car, and old worn/dirty seats make a terrible impression when buying or selling. On the other hand, great seats can really lift an interior and increase the value if you're trying to make it stand out. The original seats in my 1993 BMW 850Ci have almost 23 years of use on them - the driver seat, as pictured, looked truly horrible! It's not just dirt that makes a seat look like that though - in the closeup of the bolster you can see the real problem is the leather has dried out and the top color coat has flaked and faded inconsistently around the grain.
No amount of cleaning will bring this seat back, instead you need to source a product called leather dye. There aren't many suppliers around (I imported a small bottle from Europe for $38) but you should aim to get a perfect color match for your car. BMW leather interiors actually have dye codes like the paint codes you are probably familiar with. Finding out what you have can be a lot harder though - they are often marked on the original build or option sheet if you still have that, or in some cases a dealer may be able to tell you from your VIN (the VIN itself does not have the code - it's just used to look up the BMW database). The official BMW option for my 850Ci is LEDER NAPPA/PERGAMENT HELL (0358/M5PH) (nappa leather/light parchment). If you can't find your code, snip off a small square from under your seat and the supplier can usually perform a color match.
As with paint work, preparation is important. Clean the seats to remove all dirt - but also make sure any grease or wax is removed. I used automotive pre-paint solvent for this (same as you may use before spraying a car exterior). The leather should feel rougher now, and a little light sanding could also be done if warranted.
Now the dye can be gently sponged on to the leather. Use many thin coats to build it up, eventually working your way into every corner. The dye dries pretty quickly, but leave it overnight to be sure, then apply several coats of leather conditioner the next day (the kind you can buy in any auto-store) to soften the leather and build up a good top coat protection for the dye. It won't hurt to apply a few more coats the day after and the day after that.
Hopefully, you'll be happy with the end results! My seats are not as good as new, but they are so much better than before you'd never believe how bad they had been. Once again I can look forward to opening the car door.
