Mercedes M Klasse Rear Window Heating Problem

I have a similar Mercedes M Klasse and had a similar rear defroster issue. The auto defrost wasn't working either. Check the fuses for the rear window heating element. It's a simple check. Have you checked the battery voltage? Low voltage can cause weird electrical issues.

Can you be more specific about the fuse locations for the rear window defroster circuit? Also, what voltage level is unacceptable and indicates battery degradation?

I don't recall the exact fuse locations, but the owner's manual should have a diagram. Anything below 12V when the car is off could indicate a problem. If you're not comfortable with electrical testing, a workshop diagnostic might be worth it to pinpoint the cause.

Following your advice, a technician confirmed battery degradation. Replacing the battery restored full functionality to the rear window heating system and resolved the error codes. Thanks.