4 Speakers, 2 Channel Amplifier, and Odd Wiring

I picked up a older car that has a old custom car audio installation done years ago back in the mid 90's. I'm just going through it and replacing some speaks that had failed. In doing so I noticed a wiring setup I haven't seen before in Car Audio. I wanted to see if anyone has used this in the past and what the point and if it worth keeping like this or changing? Two of the speakers that had failed are the ones that share both channels.

See the attached image for a sketch I drew of the setup. Its 4 Speakers on a 2 Channel Amp.

The 2 in the middle of the diagram, are they subwoofers? Are they mounted in the center of the vehicle?

Yeah they are.

That wiring reminds me of the old days when you ran an amplifier in mono and stereo. What seems to be missing are inline filters (capacitors) for the far left and far right speakers as these are probably full range speakers and then another filter (coil) for the center two speakers wired in series and from your statement are woofers. The inline filter for the woofer would allow the center two speakers to go from 20Hz to let's say 200Hz and then the filters on the far left and right speakers would allow the speakers to get 200Hz to 20KHZ. With passive filters the amplifier would be seeing just the woofers at low frequencies and the other speakers at the higher frequencies to where the load (ohms) are within reason for the amp to handle them. With no passive filters on the speaker wires, then the amplifier is probably seeing a load too low for it to work right.

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1968 Chevy II Nova Garage Find 2012
1973 Nova Custom
1974 Spirit of America Nova
1973 Nova Pro-Street