What is the rating on it?
Looks similar to what I ended up running, the cermaic block is what came with the taillight, I believe the aluminum'd heat sink where the same rating, 20 watts, 5 ohms (that could be wrong now, I'd have to go out there and check now).
I ended up running two of those resistors per turn signal, one still flashed a bit too fast for me. So that was 4 total, luckly they where under $1 a piece. Wish I would have thought about hooking them up like above, I'll probably do that when I get a new taillight (found one that I like better). Looks like the resistor above has diodes inline to prevent it from turning both turn signals on at the same time?