OEM is only a two channel stereo.. left and right.. Aftermarkets are front - rear.. left-right. The left - right channels from the HU "supposedly" splice to "four channel" "simulated" input to the amp about 12"-18" inches up the wires going into the amp. The amp cuts high frequencies to the subs and outputs to left and right. Clipping the sub inputs from the harness and wiring ( "run new wires" )your "rear channels" from the HU to these inputs will then give you "front - rear" balance control.

A search on the subject will bring up diagrams etc. for doing this.

Disclaimer: this is in laymans terms.. no desire to research frequency harmonics.. vibration interceptors..passifier rectifiers or otherwise..

Kinda like RCA or Speaker level.. ain't nobody heading to the Stereo Sound-off Competition and BBQ here..