• Factory Head unit or aftermarket head unit.
• Factory speaker wiring.

These diagrams are specifically aimed at a simple factory replacement/upgrade that will not require additional wiring or stereo system upgrades.  The OPTIONAL resistor allows wiring multiple 4 Ohm speakers in parallel by adding 2 ohms back to the reduced impedance and will keep your factory stereo safe.  When you decide to upgrade to an aftermarket amplifier simply remove the resistor to produce the 2 ohm impedance the amplifier wants.