There should be a rubber coated connector for the headsets. Location varies but they usually have a clip to hold them in place and a rubber cover over the end. Left dash near the glove box is most common. There would need to be another in the rear. There may also be a rear control with a volume knob and PTT switch but not always and not needed to make it all work.
If you can’t find them, they may be tucked away as a previous owner may never have used them or they have been unplugged and lost. New ones can be bought thru a Honda dealer (local or online) or aftermarket supplier if needed.
You will need two headsets (obviously) to use the intercom. I am using J&M in three helmets and have been satisfied. But there are several other excellent manufacturers and I’m sure other experiences will be posted shortly. These have all been around for a while and they all do a great job at minimizing road noise and offering a good service. Shop around or even watch for used helmets with the headsets already installed. Wingstuff just had a sale this weekend on J&M. All Goldwings use the same connector. Bluetooth is also possible for more money.
As far as operation goes, the radio itself has five round knobs and a number of buttons. There is a button on the radio face that toggles between headsets and speakers. A light will show on the center display to indicate when headsets are turned on. This is for the radio of course as there is no need for a headset switch for the intercom.
Anyway, five knobs. From the left, first is On-OFF and volume for music, then ON-OFF and volume for the intercom, IC MUTE, AVC and TONE. The music and the intercom will each work with the other on or off. The intercom will mute the music when you or the passenger is actually speaking. The intercom is VOX and does not require any buttons or action on your part once adjusted. Radio operation is straight forward. The only thing unusual is the AVC. The volume increases with road speed. The AVC knob controls how much it changes relative to the base setting at idle. Start with this in the 12:00 position and adjust from there.
With Radio on and in headset mode, turn the intercom on and set the volume at around 10:00. The IC MUTE knob will determine at what voice level the VOX will engage the intercom and mute the radio. You can hear the music go away in your headset and your voice come on so you can do preliminary settings alone. Then of course you need the fine adjustments with your partner. If equipped, the passenger volume control only makes fine adjustment. The passenger PTT is for the CB only and has no effect on the intercom.
You will like the GL1500. We always want to see pictures. Hope this helps.