You can also make a battery harness from a spare Battery Tender pigtail, but do note that the POLARITY swaps because of the gender change, so you have to cutoff the ring eyelets once you mark the red minus (-), the black plus (+).
You can then plug it into the existing "business end" of the battery harness with the links properly going to the battery. Do remember that this outlet will be live all the time unless you put a switch in series if you leave your GPS or any accessory device ON and possibly run down your BATTERY.