You may be able to piece something together with various components if you understand airplane intercom systems but it really isn't worth the trouble in my opinion. Rugged Radios did all the legwork and put it all together for you.
And, you do not need the radio, the intercom works standalone without it (and will be radio ready if you ever want to add it). I have a Rugged Radio 4 position Bluetooth Intercom (listen to music, answer phone, etc.) and headphones (all new in box) for sale, all you would need is a rear cable sets. They are available separately from RR.
I have these items if you are interested. I have Over the Head units but only 3 of them.
| | List | Discounted |
Model 696 Bluetooth Intercom | 696-2P | $ 875.00 | $ 657.00 |
4 X Ultimate Behind the Head Headset | H22-ULT/H42-ULT | $ 876.00 | $ 660.00 |
Shipping FREE in lower 48 states. | | | $ 0.00 |
Total | | | $1,317.00 |
Also note that this system is voice activated. It is normally squelched and when someone begins to talk, it reduces any background music if playing and allows you to converse with the other passengers but if you don't have a full windshield, the wind will trigger the mics as you speed up and it is quite annoying. If you have a half windshield, it takes a lot more speed to trigger them but it still does it.