This video show how the Mercedes cabin air filter replacement for a GLS 450 SUV. This method takes only about 12-15 minutes. There are a total of 3 cabin air filters. I prefer Hegst brand when available. Here are the Amazon affiliate links for all 3:
Pollen Filter: https://amzn.to/3RBqR3T
Fine Particle Filters (2-pack) https://amzn.to/3xhav8L