Introducing the Fender-Mounted Light Bracket, a nifty way to affix a rear light onto your fender! We've stocked the E3 light mounts from Frame Builder Supply for a number of years now. Last fall, Igor and I were talking about light mounts specifically to mount to the rear fender. A lot of us ride with generously sized saddle bags without quite enough room for a seatpost mounted light. Alternately, if you use a rear rack top bag, the bag will likely cover the light. We find that mounting the light elsewhere like a dropout or seatstay is too exposed, or will be covered up by panniers.
An added benefit to this setup is the ability to run multiple rear lights. Coming from a randonneuring background, I always had two battery powered lights. For night time riding, having two rear lights really helps. You always kept a spare on hand in case something happened. You could also set them up in different positions, which I found made me more obvious to drivers (I always ran them in solid mode - being in a group of riders with a blinking rear light was seen as impolite as the riders behind you would have their night vision ruined by the constant blinking and power of the light).
I think it looks really cool and certainly makes for a more classy/elegant mounting solution.
Cost is $15.50 ea and can be found on the site here. They're currently only available in Silver.