While looking for LED lights to upgrade my trailer with I found these LED flashers for $5 at Superbrightleds.com. They flash rapidly four times, slowly four times, then stay solid. I bought one for the trailer and the bike.
Installation is simple.
The wires for the spoiler brakelight are also available under the seat, but I'd rather have the flasher safely tucked away in the trunk lid. Here is the brakelight flashing. Not bad for $5.