So two of you mentioned the backrest slot. Are these stock seats? I do not have such a slot on my 2012 seat.
So far I think all the folks responding are riding Touring models, not Softails. That may account for the confusion.
If your dealer installed the H-D brand pigtail, they only come in one length. Take a look and see if they didn't tie off the excess or route it in such a way as to make it seem too short.
Do you want the connector solidly mounted in an easily accessible location so you can just pop the cap and plug in the charger, or do you want it completely hidden when not in use? I only ask this because some folks around here are more into looks than they are function. On my Touring model, I fished the pigtail through a hole in the metal frame gusset plate located directly behind the engine, then mounted it pointing out the right side with 3M moulding tape. Your frame is set up differently, but there are many other surfaces you could use on a Softie. IMHO mounting it solidly is much better than just letting the connector hang off the end of the harness. And if you prepare the surfaces correctly, the moulding tape will do a good job of keeping the connector in place without resorting to wire ties or other methods. Mine has been hanging in there for about six years with no problems.
Jerry
FYI - the attachment is the Harley recommended method from the instructions that come with the pigtail harness. Click on it to enlarge.