I Yeah I believe those boxes are called speaker level converters.
You splice into your back speakers, run wires from your back speakers to the box's input, then you run RCA cables from the box to your amp. Good to go.
I picked up a scosche one from Walmart and it works just fine. Solid voltage, no feedback noise at all, small size and hides away nicely in our trunks.
Because you spliced into the speakers it is going to send your sub amp a full range signal. Make sure. To properly tune the LPF before turning on the system, happened to me lol
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