Where do you get a diagram of the fuse box for a 1977 Mercedes 450sel?

You need a shop manual to get that information. If you want to know what goes where with regards to fuses, take off the fuse cover panel and turn it over. It lists where each fuse, and what it controls. It also tells you the size of each fuse that's needed. Of course you know that the fuse panel is in the passenger side directly underneath the glove box.