I ran into this problem and finally got around to addressing it tonight. I was just about to click "buy it now" on ebay to order a new switch when I decided to follow JuneBug's advice and pop out the switch panel. It is just as easy as said above to pop it out. I didn't have a voltage meter, but I wanted to check to make sure that it was still plugged in. It appeared to still be plugged in, but I un-plugged it and re-plugged it in just to be safe and make sure of a secure connection. Afterwards it still didn't work, but I'd still recommend at least doing this step in trouble shooting if nothing else works.
Now that checking the plug on the switch didn't fix my solution I knew I had nothing to lose and was getting ready to order a new switch so I decided to "mash" the directional control buttons. I do not mean slam or smash them. I just pushed down on them firmly in a circular motion "grinding" them around. I did the same with the left/right switching going back and forth. IT WORKED! Both mirrors are fully functional.
My best guess is that the switch got "stuck". Hopefully, I'll get lucky and the switch won't completely fail soon, but even if so it won't be a big deal now that I've been into the "guts" of the panel. If my solution above doesn't work, don't give up! You can buy a new switch on ebay for $15 and it can be easily replaced in 10 minutes with no tools or technical knowledge. You'll see how easy it is once you pop the panel out using JuneBug's advice above.