Outer Limits 4x4 Board

Hi there,
Got a problem with my trailer.

Indicators and brake lights work fine, but the tail lights don't work at all. I know a bit about electronics but this one has me stuffed.

When trailer is unplugged the correct pin to power the tail lights sees a good 11 odd volts no worries. But when plugged in the pin shows 0V. I removed all the tail lights on the trailer (both dual element on the brake light and a single bulb over the rego plate) and sure enough we see a decent voltage at the hot end of each bulb point. But as soon as I place a load on the circuit (i.e. reinsert any of the tail bulbs) back down to zero.

I check the wiring on the trailer, all seems OK, there's a solid earth (white wire) from the plug run back to where the lights are at the end of the trailer. The tail wire (brown) goes straight back and gets split into three for the dual element bulbs and the number plate light so I can't believe their is a bad joint causing the voltage to drop prior to getting to the bulbs.
The trailer is connect to a 100 series Cruiser, I'm beginning to think it might be something on the Cruiser but it's tail lights work fine, with and without the trailer connected.

Any thoughts appreciated.

You have a high resistance joint in the vehicle wiring to the socket - as soon as you try to draw current you're dropping all your voltage over the bad joint, and none across the bulb filament.

Start tracing the wire back from your trailer socket.

You have a high resistance joint in the vehicle wiring to the socket - as soon as you try to draw current you're dropping all your voltage over the bad joint, and none across the bulb filament.

I've had 2 plugs/sockets with this problem. The actual plug was the problem, visually it was perfect. Just replaced the plug and it worked a fine. Cheap enough to replace both plug and socket, even if its not the problem.

Thanks guys, I'll start tracing it back through the vehicle