Caravan road lights problem

[Kenty]

Anyone out there who might know the solution to this problem?
Side lights, indicators and brake lights all work individually but when two are used at same time all the bulbs light up - for instance, with side lights on and brake is pressed the side lights dim and the indicators also light up! Side lights on, indicator used and side lights also flash. It seems to me there is a track back to neutral/earth somewhere, am i correct?

Tried so far -
all light fittings removed and checked for water/tracking, all clear - no difference
new caravan 7 pin plug fitted

[Bill]

Sounds like a poor earth connection, possibly in the tow car.

[ColinM50]

Definitely a false earth. Check trailer and car

[marshal_alan]

deffo a short somewhere, get the multimeter out, would try plugging the car into another trailer or caravan to eliminate the towcar. be warned this can be a right pain to sort

[Nigel Worc's]

marshal_alan said:

deffo a short somewhere, get the multimeter out, would try plugging the car into another trailer or caravan to eliminate the towcar. be warned this can be a right pain to sort

Unlikely to be a short as they wouldn't work individually, it is much more likely to be a bad earth (it isn't really earth, it is the 0 volt return), as others have said.

[Kenty]

Thanks for info, still working through it, all good advice. Your right it is a right pain.

[Tunku]

If you have any dual filament bulbs in eg. rear light/brake light combo, check they are correctly fitted, plus - check both filaments are ok. I had one of these bulbs on a motorbike and the bulb failed with one filament touching the other which meant when I pressed the brake, my headlights came on.

[Kenty]

Thanks Tunku I'll check that, there are two of those.