The fuse I mentioned is not in the regular fuse box. Also, it doesn't look like a fuse you see in the fuse box. It is a strap of metal about 1/4" wide and and inch or so long with a screw at either end. It will probably be in a small black plastic box barely large enough to hold the fuse.
Unfortunately, much of my information that I gave you in my first response may not apply. I was making the assumption at the time the you have a 123 car with which I am somewhat familiar. Your 115 diesel is different. I'm sure that there are some people who are familiar with the 115 that can help. I would suggest starting a new post with a title that makes it clear that you have a 115 diesel car.
As far as a "tuneup" goes, a tuneup is sort of an obsolete operation with modern cars and with diesels. A "tuneup" on a diesel really only involves changing fuel filters and maybe valve adjustment. What I believe you need is repair of your glow plug system. You might put out a post to request a good independent MB shop in your area.
Since, as I said, I'm not familiar where the glow plug relay is located on your car, removing the instrument cluster would probably be pointless until you find out precisely where the glow plug relay is located. However, the instrument cluster on your car is mounted in a rubber surround and just pushes out from underneath. Reach under the dash, behind the cluster and push it toward the steering wheel.