most night lights are designed to attach to the seatpost. but while carrying a bag on the rear rack that area get's blocked.
some manufacturers sell cheap, awful, lights that attach to the rack (topeak) but i figure some kind of adapter would be much more useful (mostly since everyone may have night lights before getting a rack... i do :)
but my searches for such a adapter failed. do you know any?
This is one example of night light i have http://www.dealextreme.com/p/rechargeable-7-mode-red-5-led-safety-bike-tail-light-with-7-mode-2-red-laser-beam-35mw-90117 (i regret. do not buy, laser is useless snake oil. but the leds are OK. EDIT: regret #2, i had rain for the 1st time --socal-- and when wet, the shorted the buttons which are obviously not water proof, so it spend all night turning the leds and laser randomly on and off. battery hold up ok though.)
and this a rack http://www.blackburndesign.com/racks/mtn-2-rack.html (it has a hole on the back, where you can fit a L shaped adapter that holds a reflexive cat's eye)