Outer Limits 4x4 Board

hi all
i recently put a trailer socket on my sierra and i have a problem
the indicators work and the brake lights work but when i put the indicators on and put my foot on the brakes the indicators on the trailer stop working and the brake lights flash.
the trailer wiring isnt broken as it works fine on another car
ive redone a couple of earth wires also
and the reverse lights on the suzuki dont work but i dont think this would cause this problem.
any ideas

One is earthing through the other. Do you have a dedicated earth wire in your trailer socket, or are you relying on chassis earth through your towball/towbar? (Which is bad, mmkay?)

Some other possibilities:

• Sierra isn't wired correctly - how did you choose which wire went where?
  "Other" car isn't wired correctly, so trailer is incorrectly wired to suit incorrect car.

Do you have/can you use a multimeter? Check the trailer plug pins for continuity - use low ohms scale. Each pin should have lowest ohms measurement to your earth pin, and resistance to any other pin should be higher than to earth - measuring through two filaments in series, instead of one.

Good luck,

Scott