Lighting & Electrical
Trailer lighting isn't optional trim — it's a registration and roadworthiness requirement, and the plug type has to match what the tow vehicle actually carries.
Required Lighting
- Tail lights — visible from behind in low light/darkness
- Brake lights — illuminate brighter when the tow vehicle's brakes are applied
- Indicators — synced to the tow vehicle's turn signals
- Number plate light — illuminates the trailer's registration plate
- Reflectors — red rear, amber side, passive visibility even with lights off/failed
All trailer lighting must comply with the relevant Australian Design Rules (ADRs) covering external lighting and reflective devices.
Plug Types
The two common Australian standards are 7-pin flat and 7-pin round (large or small round pin variants also exist on older vehicles); heavier trailers with electric brakes typically step up to a 12-pin plug to carry the extra brake controller circuit. Mismatched plug types between tow vehicle and trailer are one of the most common reasons a lighting or brake connection fails intermittently — an adaptor is a workaround, not a permanent fix, since adaptors are a common failure point themselves.
LED vs Incandescent
LED trailer lights draw less current, respond faster (relevant for brake lights, where a fraction of a second of extra warning matters), and last far longer than incandescent globes — most modern box trailers now ship with LED lighting as standard. Incandescent globes remain compliant and are sometimes retained on older or budget trailers, but need more frequent globe replacement over the trailer's working life.
Wiring & Brake Controllers
If the trailer has electric or electric-over-hydraulic brakes, the tow vehicle needs a compatible brake controller wired in — this is a separate circuit from standard trailer lighting and typically needs the 12-pin plug standard rather than a basic 7-pin. Trailers fitted with mechanical overrun brakes (see the braking systems guide) don't need this — braking is entirely mechanical and doesn't touch the electrical system at all.
Questions & Answers
What plug does my box trailer need?
It depends on the tow vehicle's existing socket and whether the trailer has electric brakes. Unbraked or mechanically-braked trailers typically use a 7-pin plug (flat or round, matching the tow vehicle); trailers with electric brakes generally need a 12-pin plug to carry the brake controller circuit.
Are LED trailer lights compulsory?
No — incandescent lighting remains legally compliant. LEDs are simply the practical modern standard because they draw less power, respond faster and last longer, not because incandescent lighting has been outlawed.
Why do my trailer lights work intermittently?
The most common causes are a mismatched plug/adaptor combination between tow vehicle and trailer, corroded pins, or a damaged wiring loom — check the plug connection and adaptor first, since that's the most common failure point.