d23 trailer plug mystery

[gossy]

Hi .
This is my first time on a forum.
Wanting some help with a trailer plug problem. All the lights work correctly on my trailer as i checked it on my wife car.
and individally they work on my 2018 rx navara . the problem comes when i have the sidelights (park) on and the brake lights fail to come on. but they work fine independently.
Any help would be much appreciated.
thanks Gossy.

 

[Old.Tony]

I'd suggest there's a poor earth involved. It'll be either:

* A fault in the tail light units themselves. My left rear one needs to be opened and cleaned now and again, otherwise it dicks around with indicators and reverse as well.

* A fault in the earth wire for the plug. In a flat 7-pin standard plug. This is usually the centre pin. If you get a multimeter and test continuity between this pin and the vehicle chassis (say, one of the mounting bolts for the tow hitch) you should see zero Ohms. If you see anything else, the earth wire isn't connected properly. Auto electricians often connect this haphazardly via a small self-tapper to anywhere they can easily stick it. Not the best move!