I have a ute (Australia, so truck to the Americans in here) and a car trailer.

I have a 4 LED lights on the trailer wired to a relay which is powered by, and earthed directly to, the trailer-mounted battery (which is not grounded to the trailer itself).

I am attempting to use the 12 V signal from the reverse lights of my ute, as well as the park lights from the ute, using a SPDT switch.

Question is, do I need to ground the relay to BOTH the battery on the trailer AND the truck, in order for this to work? This could be as easy as grounding the trailer battery to the trailer itself, which would be grounded the the truck on contact (in theory).

It's late here atm, and I can't get to the trailer to check this theory, but it wasn't working when I tried to get it working a couple days ago.

The lights work when using a wire to the signal pin on the relay from the trailer battery, but not when I have the SPDT switch wired with the 12 V signal from the truck lights.

  • 1
    \$\begingroup\$ A Ute is closer to a pickup. As with all electrical problems, draw a schematic so we have something concrete to work from. \$\endgroup\$
    – Kartman
    Sep 6, 2022 at 11:05
  • 1
    \$\begingroup\$ The relay's coil will need the ground from your ute, because that's where the control signal is coming from. It doesn't need to see the trailer's ground. \$\endgroup\$
    – brhans
    Sep 6, 2022 at 12:00
  • \$\begingroup\$ Close voter, how is someone attempting to design an electronic circuit, a use question \$\endgroup\$
    – Passerby
    Sep 6, 2022 at 17:38
  • \$\begingroup\$ You legends! Thank you so much for clearing this up for me. I didn't realise that the coil needed to be ground to the same circuit as the signal. MUCH appreciated guys 👌 \$\endgroup\$ Sep 7, 2022 at 0:42

1 Answer 1


Here's how.

enter image description here

You need to connect the trailer battery negative terminal and a terminal of each lamp to the trailer chassis to save on wiring.

The fuse on the trailer battery positive line is not shown.


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