Last year I installed a 12v ciggy outlet in the left-hand knee pad. Works just fine except that on bumpy roads, such as my three hour ride to Richmond this past weekend, I found that the GPS plug kept vibrating out. I'd get the "Power down in three-tenths of a nano-second" message.
The fix (well, fudge)? I found that if I put a bit of tension on the power cord when I wrap it around the RAM mount it holds the plug in place. I can't put too much, though, as the handles bars have to turn.
There must be a better solution.