Electrical Help Installing Amp

If everything is dead including the headlights, then the problem HAS to be between the battery and the battery junction box.

Confirm the battery voltage with a real volt-ohm meter.

Check the white fusible link in the main battery cable. There should be +12 volts at the main B+ alternator terminal with the key off.

Confirm the negative battery terminal is clean and tight as it is bad about splitting when tightened.

Confirm that the main battery cable is connected, clean, and tight at the BJB.

Confirm there is +12 volts to the input side of the fuses in the BJB.

I have this problem with my truck sometimes. The negative terminal likes to lose contact. The terminal will be tight on the post but due to some build up or moisture it needs to be cleaned and reconnected or it wont ground. I'll get in, the dome light will come on, turn the key, and whole truck goes dead. Hopefully yours is nothing major.