Application specific bracket and hardware Only needed when you are upgrading to a 2nd subwoofer in a Cam Am X3 The bracket and included hardware allows three MB Quart amplifiers to be mounted in the X3 (1 full range and 2 mono) To clarify, this 3rd amp bracket is only required when running two subwoofers Purposely Engineered X3 Tuned Audio Mounting System Amplifier 3 (2nd mono amplifier for subwoofer system). SUBWOOFER ADD ON KIT X3