97 E420 Cabin Air Filters
 

Do I get at these by taking out the glove box, or are they changeable from the engine compartment ...please?

Glove box.

How do you know when it is time to change these?

We do them at the B-Service and they usually come out fairly dirty. It only takes a couple minutes to check them, though. With the glove box removed you'll see a rectangular access door held in olace by a white clip. Slide the clip up and remove the door. Both filters ( identical ) can then be removed. Keep in mind that the glove box can be a bit stubborn to remove, especially if it's the first time it's never been done. Also, make sure the access door is correctly re-installed or you'll wind up with a whistling noise at high fan speeds. Good luck