Since your intercom harness is not stereo, then you can simply bridge the sender and ring terminals. The ground is the tip terminal. Just for positional reference, the terminal ring closest to where the plug is inserted is the tip terminal. Next is the ring terminal (in your case). Last is the sender terminal. Personally, I'd return the stereo jacks and get the mono jacks to match your intercom. The headsets won't care, the plugs work in either type. Playing devil's advocate; if you keep the stereo jacks you won't have to replace them if you go with a stereo intercom at a later date.