I changed the cabin air filter at around 60,000 and it was loaded with crud.
If you look under the dash, to the right of the gas pedal, you'll see a little white plastic door. The filter is inside that door and it is in three pieces that interlock with each other. It is a royal PITA to change. You slide one out at a time, then you slide the new ones back in one at a time. Good luck.
Did you look to see where the little door for the filter is yet? If you did, you'll see what I mean. The filters themselves are made so they bend. You can't put them straight in, you have to bend them at around a 30 degree curve to snake them in and out. The gas pedal and the shift cable are in the way.
i changed the cabin filter every 2 years on our 1999 sls.
a real difficult job.
my dealer charges $40.00 for labor.
i supply filter about $30.00
well worth money!
ask for old filter to be returned to you!