If you're at all good with electronics, just do it yourself. Put mine together in about 2hrs with a handful of LEDs/resistors and some scrap 1/4" ABS.
There's 3 lines. One is power (+12), one is ground. The third is the signal for the brake light. On the 900s, that line is high relative to ground (+12) when the brake light is off. That line drops low (0.7v-ish) when the brake light comes on. On other sleds, that logic is opposite, where when the brake light comes on, that line goes high relative to ground. The "adapter" you buy is just an inverter that toggles that signal line the other way.