Tow bar wiring - earth problem?

adamjack
adamjack Forum Participant Posts: 1
edited August 2013 in Caravans #1

I have just bought a new (very old) used caravan and took my car in to get a tow bar fitted.  I have checked the output of the electrics on the car with a meter and tested the input to the caravan with a 12V battery and both are responding as expected. 
When they are hooked together everything works great except the brake lights.  The brake lights come on when the battery is put directly into the caravan plug so I know they work and the the wiring is correct all the way through.  A friend suggested there
may be an earth problem but I don't know where or how to check/fix that.  Thanks in advance for any help!!