Use this article if…
- You need a reliable reverse +12 V sense (PWM tails, taps, relays), not lens cleaning or video format.
- No switch on screen → No reverse display. MK4 → MK4 no reverse display.
Quick answer
Headunits that never see reverse are very often tapped into a PWM or current-limited tail lamp that never reads as solid +12 V - meter the trigger at the radio harness in reverse and move to a relay, body pin, or integration output if the diagram says so.
Problem
- Reverse gear must deliver a reliable +12 V sense to the headunit.
Symptoms
- Reverse camera input never switches when selecting reverse.
- Multimeter shows odd or pulsing voltage on a tail-lamp reverse tap.
- Behaviour changed after trailer loom work or LED tail upgrades.
Cause
- Many tail-lamp circuits are PWM-filtered or current-limited - your headunit may never see a solid +12 V reverse sense.
Fix Step-by-step
- Confirm reverse lamps work; then meter trigger at the headunit harness in reverse.
- Avoid PWM-only tail taps - use body pin or relay output if the sheet says so.
- Assign the correct camera input in headunit settings.
- Boot-cycle; test with parking brake set and wheels chocked.