I found mine a few months back when I installed the Harley cigarette lighter type power accessory in the trunk of my Ultra. I'll try to attach a picture of the accessory plug you are looking for. It was in that mess of wires and harnesses between the battery and the tank....like Sickbagger said. There's more than one plug in there, at least on my bike there is. I used the black plug instead of the gray plug....different plug type. Both of these connectors have rubber plugs in their ends.
The plug you are looking for is the one on the left side of the picture just under the white wire label. I've got the Harley dual accessory wire harness plugged into it and you can see the rubber plug underneath the wires.
VR
PS After plugging in the cigarette lighter jack into the dual accessory harness I didn't have power. After much consternation and troubleshooting I discovered that the pins on the dual accessory harness plugs are wired differently than the plug on the bike. It's easy to reconfigure the pin wiring to match up correctly when you know how.....I found out how in my shop manual. Let me know if you need help with that.
This reminds me of the battery tender lead that is supposed to be in my bike somewhere ....... I still can't find that sucker after nearly two years of trying.