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

[Kenty]

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

[Kenty]

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