A box with a couple of fuses outside of the main fuse box is probably the AC blower fuse box.

If you remove the cap over the glow relay (behind the left headlight), you'll see a couple of plug in connectors and an inch strip of metal held by two philips head screws. The metal strip is the 80A glow plug relay fuse. At least in the 300SDL.

Sixto
91 300SE
87 300SDL
83 300SD