I have the Mars 2.0. It has several mounting options. One is to mount in a regular reflector bracket. Since a lot of racks have the reflector mount, you can put it there if your rack has it.

Oddly enough, I have a Topeak rack on my bike, which has a bracket only for their Topeak light. Even more odd, I mounted still another light instead, the same as the big fat one on the back of 10wheel's bike here:
http://www.bikeforums.net/showthread...=437032&page=3
The Mars 2 is an o.k. light; I use it as a blinky, and emergency back-up. The big one really stands out; I highly reccomend. A couple angle brackets and a few minutes, and you could probably make it stick to the rack. I think 10wheel just stuck his to the trunk bag. Good luck.