Got a '05 Touran. All the caravan road lights work except the brake lights. They work on the car and they work on the caravan when tested with a tester or another car.
Is there an isolation module on the car maybe that particular part of the module /relay isn't working on the car as most modern vehicles you cannot wire the 7 pin socket straight to the lights so if the lights and 2iring work on the caravan and brake lights on car work the only thing between them is the 7 pin socket and module
Possibilies,
Check fuse, ?good connection in light cluster, clean the contact points on the caravan bulbs, clean the caravan earth connections in the light cluster.
First thing to check is connections on 12n towbar socket on car. It might be as simple poor connection to brake light pin which is pin6 red cable. Or poor earth. If inside of socket is corroded its best to replace socket & strip back wires to clean metal.
I had the exact same thing. Its the wiring on the towbar.
The testers have a big design fault... They use LEDs which require a tiny amount of power.
A single strand of wire will light the LEDs and show the towbar socket as all good but if you put a normal bulb onto the wire it wont carry enough power to light the bulb.
Check the towbar wiring, make sure you put a load onto the wire being tested.