Use this article if…
- The headunit never switches to camera in reverse (any aftermarket unit), not “camera page opens but stays black.”
- Black image after switch → Black / wrong picture or MK4 reverse black screen.
- MK4 - no switch at all → MK4 no reverse display. Sense wiring → Reverse trigger wiring.
Quick answer
If reverse never switches, the headunit is not seeing a reverse trigger. Confirm reverse lamps work, then measure the trigger wire at the radio end while an assistant selects reverse.
Problem
- Reverse gear does not switch the display to the camera input.
Symptoms
- Reverse lights work but screen stays on radio/map.
- Diagnostics show “no signal” or timeout.
- Worked until trim went back on - now intermittent.
Cause
- No trigger to headunit, missing or incorrect reverse-on wire.
- PWM or filtered tail circuit, meter shows odd voltage; headunit never sees solid +12.
- Wrong input selected in headunit (camera 2 vs camera 1).
- Fuse or join failure at the rear harness.
Fix Step-by-step
- Confirm reverse lamps illuminate; then measure trigger wire at the headunit end in reverse with parking brake set and safe conditions.
- If trigger is unreliable, re-run per your integration sheet - often a dedicated body pin or relay output is cleaner than a splice guesses.
- In headunit settings, assign the correct physical port to “reverse camera”.
- Boot-cycle and test in Park vs Drive where applicable.