While the unit is off, disconnect the bass knob. If it then works, the bass knob is the problem. You can run it without one, or buy a universal bass knob for like $20.

If it doesn’t work... Leave the bass knob disconnected.

Connect rca’s to the sub from something else that you know works. Like a home stereo connected to an extension cord out to your car or something.

If it doesn’t have sound, the amp/sub is the problem. Take it apart and make sure the speaker wires are connected between the amp and speaker. Might be that, might be a dead amp.

If it had sound, your sub isn’t the problem. Reconnect the rca’s from the car, and disconnect them from the head unit, then connect to the receiver/test device you know works.

If it has sound, your head unit is not putting out a signal over the rca’s. It may be a setting in your head unit, or a defective head unit.

If it doesn’t have sound, replace the rca cables.