Rear Seat Expansion Kit, designed for extending a two-person intercom system to a four-person setup, includes options for headset and helmet kits suitable for both STX Stereo and Mono Intercoms. Features a 16' cable for back seat accessibility. AlphaBass Headset features true Carbon fiber, flexible boom mic, leather ear seals, and M101 Red dynamic noise cancelling mic, along with convenient volume and bass control. Ultimate Mono and STX Headset comes with Carbon Fiber style finish, M101 red dynamic noise cancelling mic, comfortable gel ear seals, and handy volume control among other features. Mono and STX Helmet Kit features Alpha Audio speakers, Clear Hear speakers, dynamic noise cancelling mic, and is easy to install with peel-and-stick adhesive. Compatible with STX Stereo high fidelity intercoms and Rugged Radios MONO intercoms.