The APP0261 allows you to retain the factory 4 channel amplified system in your Toyota or Lexus when upgrading to an aftermarket headunit. The harness simply plugs into the factory 15 Pin Lexus/Toyota plug behind the original headunit and converts it to 4 RCA plugs which connect into the rear of the aftermarket headunit that as 4 RCA amplifier inputs (front and rear). The harness also provides several bare wires which need to be soldered to the aftermarket radios wiring loom including Constant Power, Accessory Power, Amp Turn On Trigger etc. (wire details can be found in specifications section). When using this harness the Aerpro ISO Plug and Play System will not be compatible.