These adaptors allow you to install an aftermarket amplifier to your factory head unit. These devices take the speaker level outputs from the head unit and reduce the signal to a level of which an aftermarket amplifier can use without overloading its inputs. APU8SUM Complete integration of aftermarket audio components into Vehicle OEM system. Sums and balances multiple signals from Factory Stereo, sums up to four Speaker-level stereo pairs down to single stereo RCA low-level output, built in AUX input for connection to Portable Audio devices and four line-level stereo RCA outputs connect to Amplifier.