Head Unit Won't Power On

Step 1: Check constant 12V wire (yellow). Should be 12V at all times, key on or off.

Step 2: Check accessory wire (red). Should be 12V with key in ACC or ON position.

Step 3: Check ground (black). Should be solid continuity to chassis.

Step 4: Inspect in-line fuse on yellow wire and head unit fuse (often inside unit on rear PCB).

Step 5: Confirm harness adapter correct for vehicle. Wrong harness can create open circuit.