Stock savage rear signals are single filament
you're putting in duals, usually put in the front for signal and running.
harley lights are usually grounded to the frame
in dual filament lights, one is bright, the other dim.
confused yet? take a deep breath, I am.
you pressed in a single filament bulb and it works (blinks) right.
a single filament bulb has a single connector, most likely when you pressed it in it contacted both contacts in the socket... sooo one of the wires in wired correctly the other is not.
Also, w/o that third wire, ground may be intermittent.