What is the purpose of the stereo crossover box? Is it needed? Anyone have the wiring schematic or how the box is wired in a 5 speaker setup?

well…a crossover can be real simple (a capacitor that restricts low frequency (power) to delicate but simple tweeter. A more complicated crossover sends ONLY low frequency to he woofer, mid to the mid, and high to the tweeter.
Yes, they are important.

It only deals with the front dash speakers.
Balances the sounds going to the outers & that center speaker.
There are a few sites that have diagrams, just do a simple search.

It takes the left and right outputs via 2 LC circuits for a third (middle speaker) output. Since all stereos only have a left and right output you could make your own harness to connect to only 2 speakers, or 4 depending on their ohms. For 3 and 5 setups you need it.

I had one of those for years and couldn't give it away. Don't even know if I still have it or tossed it

Great THANKS

Thanks everyone. Always great information available here.