When looking over my new to me 05 softtail I noticed a plug-in sort of thing just under the seat on the right. Sort of hidden. Not knowing anything about Harley's at the time (still don't know much) I guessed it was some sort of hook up for a dealer computer or some such. I installed a battery tender and let it sit for a month or so. After getting some info here and perusing the service manual I wanted to check some things out just to familiarize myself with the bike components. I pulled the seat and removed the battery to check it and get a better look at things. When removing the battery I discovered that the mystery plug is, you'll never guess, a battery tender hook up. I'm still laughing at myself about that.