If everything else is working, I'd leave the wire disconnected. You don't know what it's for and could cause more problems if hooked up.
That type of relay is normally a spdt, one connector is NO and the other is NC. There should be a schematic printed on the relay, or get its part number and look up on line what the vacant terminal is.
Rick & Debbie; Brandy, Dexter R.I.P., & Fritz R.I.P. (The Doxie "Kids")
2015 Jayco Pinnacle 36RSQS 5'er
2011 Rockwood Windjammer 3001W (Traded)
2013 Chevy Silverado 3500HD LTZ, 6.6L Diesel Dually; B&W Goose Neck and Companion 5'er hitch