I picked up a Busch and Muller Toplight Line Brake Plus taillight, but it isn't quite the one I need. I took it off the packaging and test wired it against my Ixon IQ-X and it works perfectly when I spin the wheel (and even brightens when I slow down the wheel a little to simulate braking).
The taillight was never mounted, and is in "Open Box" condition. They're going for $66 on Amazon and $61 from Peter White. How about $45 shipped in the US?