Start the engine, and let it idle (we want to power up everything)
Use a volt meter to check both sides of the fuse (with the meter set to 20 or so volts DC, and the black lead to any bare metal screw head - ie. the body is power ground - negative), and then test first the lower side of the fuse, then the top of the fuse. If there is 12 Volts on one and not 12 volts on the other, you have a blown fuse. If, however, you find that there is apparently no power to either end of the fuse - check to make sure that the fuse is making good contact in the fuse holder (mine had several that were not when I first got it). You may need to pull the fuse to check the fuse connector (one side should have power).
If you find some bad connections - power down your Sprinter, then get some spray electrical cleaner and spray the bad connectors on the fuse box. Also look to see if you can bend the fuse holder connectors slightly so that it holds the fuse tighter (I had to do this). Also clean the contacts on the fuse.
I went thru all my fuses cleaning and tightening the fuse holders (its a poor design that they used - in my opinion). I cleaned all of the relay contacts (one relay at a time) too.