It's just plain 'ole dirty and especially stands out now that I re-dyed my leather and plastic trim with Leatherique dye. I found it at Escondido MB for $9 plus shipping
BTW, if you have a light colored interior like I do (cream/yellow [interior code 265]) and if it's starting to show it's age, (leather has cracks in dye, plastic trim pieces fading to different colors, etc) I highly recommend the re-dye process I am almost finished with. The results are amazing and WELL worthwhile. The interior looks essentially brand new.
I'll post some pics when I'm done. Unfortunately I didn't take any before pics.
'91 300E, 208k miles