If your headphones are like most then the red wire is right audio that carries signal to the right ear-cup, that being said I am assuming you have opened up the left ear cup since you see two red wires. One of those as I said earlier goes to the right can and the other should be for the source...

Hi all, I caught a bug collecting vintage cans from the mid 70s, I like to refurbish them and rotate em at the community radio station I work at. I also gesticulate a lot while talking, which explains the profile photo. I'm happy to meet ya' all!