What you'll discover when you remove that sissy bar is the rear signals mount through a special oval shaped hole in it. They were mounted to the original grab bar on the stock bike. You may need to acquire one of those. See the last post in this thread .....
http://www.xs650.com/forum/showthread.php?t=9113
Or, you could acquire some of the little chrome brackets the Standard models used to mount their rear signals .....