you're going to trace back from the fuse box to the battery trying to find where you loose power in that circuit. i had a similar problem with my 85 635, and it ended up being a pair of smoked relays. but still, a multi meter set on 20V is going to be your best friend on this one.
i usually say to start with the offending/problem part and work back toward the battery to diagnose electrical problems, but you may have something as simple as the power input wire going up to the fuse has been pushed down far enough to not make contact with the fuse!
hope this helps.