Lite Mate question

Hello,
I'm replacing the worn out plug on my trailer towing light harness. It's a Lite Mate (Hopkins?) that was on the truck when I bought it used. Looks like the plug and play harness that plugs into the tailights.
My question is: This is a four flat plug...I want to solder a generic four flat to the existing wires where I cut the old one off. Problem is, the Lite Mate has five wires going into it. The generic has four going into it. The Lite Mate plug has stamped on it's casing which wire is what; running lights, brake lights, left turn and right turn. The plug casing is worn where the 5th wire(blue) is named, so I don't know it's purpose. Without having tried it yet, can I solder the four to the new plug leaving the blue wire unused? Or is this a ground of some sorts, and needs to be tied in with another wire? I hope my question is making sense.
I know I can get a new Hopkins harness for about $25. But I was hoping to go the cheap route as I'm unemployed. Thanks in advance for any info.

Garth

 

I think you have a better connector than just a standard 4 way. You should be able to wire it to a 4 way just fine though, as the 5th looks like to be back up lights. I think this pdf should tell you where each wire should go, although blue isn't an option for the 4 and 5 way switches. Of course they could've changed wiring colors at any time as well.

http://www.hopkinstowingsolutions.com/doc/trailer_wiring_codes.pdf

As long as you get the running lights, brake lights, and turn signals wired up into the 4 way and working right, any extras aren't really needed for a small trailer.

 

Thanks Manlaan,
According to the bottom diagram on the PDF, my blue wire may be a ground, as it comes out of the plug I have as the male pin.