Thanks for the suggestion of checking the fuse. I don't know why I didn't think of that, since this IS an electrical problem, after all.
Only today did I get to check the fuse, and now I'm confused (no pun intended). My owner's manual says that fuse #4 should be the fuse for the power outlets, and it should be a 15 amp fuse. But fuse #4 in my bike is a 7.5 amp fuse. And NONE of the fuses in the bike is blown. The cover over the fuse box has a label for the function of each fuse, but the labels are all in German: the label for fuse #4 says, "Stockdoso", except it doesn't have 3 "o's". The third character in the word, and the last character, each look like an "o" with a horizontal bar through it (the seventh character in the word is just a plain "o").
To compound my confusion, there's reason to question my owner's manual. Although it is supposedly the one that originally came with the bike (and I believe that), it shows things in it that my bike does not have. For instance, it shows a tool box under the seat, but my bike doesn't have one, and doesn't have room for one (I know, because I bought a used one off Fleabay and it didn't fit; fortunately the seller bought it back from me). And it shows different side covers than my bike has. The owner's manual doesn't identify what year (or years) of bikes that it covers, it just says it's for the K75, K75S, and K75RT. Mine's a K75, 1994.
So, should I substitute a 15 amp fuse for the 7.5 amp fuse in position #4? (I'm no electrician, so I am leery of replacing a fuse with one of higher amperage unless I know what I'm doing, and I admit that I don't know what I'm doing.) And if the 7.5 amp fuse wasn't blown in the first place, would that make any difference? Sheesh.