Access is through the back of the seat. In order to get to the back of the seat, you must remove the seatback. In order to get to the seatback you must remove the lower seat cushion.
The lower seat cushion comes out quite easily by pushing-in the two orange tabs on either side underneath and lifting up.
Once the lower seat cushion is removed, you need to remove the seatback by unscrewing the three 10mm bolts at either side and the one in the middle.
This should give you access to the track of the armrest and to be able to see what is wrong. After repairing what is wrong, it isn't a bad idea to lube the armrest tracks.
If you need to remove the seatback from the car, it is a good idea to have someone help you since your car is a coupe (tighter spaces) and the two outside arms at the bottom can be sharp and easily cut interior pieces.
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)