I have opened several fuse boxes and added a lot of (benz original and after market) cables...a "floating" fuse underneath the fuses in the fuse box is definitely after market. Take a look at the other end of the cable (which goes into the fuse box) and disconnect it. When the fuse breaks whilst starting the car it is already shorted and of NO GOOD.
what type of car, what color coded cable?
with a minor 600+k