Emery Stora tells us that the green wire is for right turn/brake. Yellow is the left turn/brake. Brown is the taillights. White is the ground. The blue wire is a spare wire. It runs from behind the dash (or below it) to the back. It could be used for a break away, to bring power to a speedometer disconnect, etc. I believe that it goes to a 12 wire connector. This might be to the left side of the driver at the front of the side panel. Bob de Kruyff stated "the harness includes an extra blue wire that could be used for brakes. The wire is a bit hard to get to since it ends at a 6 way plug (I think) that is behind the side panel next to the driver. Sometimes you can reach the connector by reaching from below the dash and behind the panel. The wire is taped back in the harness as it leaves the back of the coach. It's not a very heavy wire so I'm not sure it can handle the amperage required--on the other hand I think that's what is is for. It is also blue which is the standard color for tailer brake controller output. "