Easiest way to start would be to check the inputs, I think. You need to know where the problem is.

Depending on what you have handy, you could either
1) take an adapter, a headphone jack to male RCA adapter, to use with your phone, or
2) take an extra RCA cable and pull your head unit out for a temporary connection

Option 1 is good because you know your headphone jack works - Sorry if you have an iPhone, Apple is the absolute shittiest company on the planet and it's unbelievably not just the products but the policies, finances, abuses - that's it's own exposé I don't have the hours to get into now - but eff them and their removal of headphone jacks in their profiteering onslaught with expensive mediocrity as a waste product.
TL;DR - fuck Apple - as an audiophile, this isn't the only reason you need a headphone jack.

Option 2 you have to be a little more careful because we don't know that the head unit isn't the problem! If it is, you might need to explore settings on the head unit - for example, are you using the front RCAs and someone turned the fader all the way to the rear?

The idea in either case is to first disconnect your RCAs at the amp and DSP, then plug your test RCAs into the amp directly.

The process is this-
1) turn the car on. Confirm your power lights come on in the amps and DSP.
2) Plug your test RCA into the amp channels directly. Do you get sound out of the speakers for that amp channel pair? Then the problem is the DSP or head unit.

If you used the head unit as your source - you've also confirmed the HU isn't the problem...
...unless you got no sound, then maybe the HU is the problem - time to use the headphone to RCA adapter to confirm if it's the HU or the amp are the problem.

If it's the HU - time to explore the settings in there.

If it's the amp - make sure there's no protect lights on or anything like that.

If you did (or can) feed the RCA from the HU to the amps, bypassing the DSP, and you get sound - then you've confirmed that the DSP is where the problem is.

Diving into the DSP is unique to each DSP...

...but the first step is identifying the problem component by elimination, then diving in deeper to that particular component.

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