This site describe, explain and give a good illustration about car schematics and . Start with basic system which consists of a head unit (HU) and 2 pairs of speakers, here you see that the front speakers are being driven from the HU’s internal amplifier and a pair of rear speakers are being powered by an external amplifier.

The following discussion on Car and schematics continue respectively with:

Head Unit, 1 Amp and 4 Speakers
Head Unit, 1 Amp and 4 Speakers (all on amp)
Head Unit, 1 Amp and 4 Speakers (2 subs on amp)
Head Unit, 2 Amps and 4 Speakers (2 subs on one amp)
Head Unit, External Crossover, 2 Amps and 4 Speakers
Head Unit, External Crossover, 3 Amps and 4 Speakers
Start of 4 Channel Amplifier Section
Basics of 4 Channel Amplifier
Basic System with a 4 channel Amplifier
A 4 channel Amplifier system with 4 Channel Input
4 Channel amplifier with Bridged Rear Channels
4 Channel amplifier with Both Front and Rear Channels Bridged
4 Channel amp on front speakers, 2 channel amp on subs

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