One Channel Dead
Step 1: Test with known-good speaker directly at amp output. If channel still dead: problem is in amp or input signal.
Step 2: Swap RCA inputs left ↔ right. Does dead channel follow the cable? If yes: problem in head unit/source or that RCA cable. If no: amplifier fault.
Step 3: Check crossover settings. Is HPF set so high that signal is being filtered out? Set HPF to 50 Hz or off for test.
Step 4: Check for internal fuse inside amplifier (some amps have per-channel fuses on PCB). Requires opening amplifier.
Step 5: If all above pass — internal transistor or op-amp failure. Professional repair or replacement.