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Hi, what year and model is your Mercedes?
Ok, are the fans running continuously with the key off?
If you remove fuse 18 do the fans turn off?
Try removing the relays at position E, F & G in the back section of the fuse box and see which one turns the fan off. It sounds like one of the relays is stuck closed. I've attached a diagram below.
Sorry about that. Click on the diagram below. You will need to remove the lid on the fuse box to access the relays in the back of the box.
You did not remove enough of the cover. You need to remove the 6 or so screws that are holding the lid to the top of the box in order to access the relays in the back of the box.
You have to remove the relay that mounts to the top of the box behind the fuses then remove the six screws around the perimeter of the lid and the lid will lift off leaving the fuses in place and the relays exposed. I've attached a picture below. I'd go ahead and replace the fan relays.
Disregard the reference to the K17 relay. You will want to replace the K9 relays.
The website restricts me from sharing my private email without an additional service agreement. I've sent the agreement with this reply. You can contact customer service and send them the pictures and have them forward them to me if you prefer.