Vauxhall Frontera 1995 2.2 - trailer brake lights do not work?

Vauxhall Frontera 1995 2.2 - All lights work on trailer except brake lights. Have tried trailer on another vehicle and all the trailer lights work ok, so must be something wrong with the Frontera. Where are the fuses on the Frontera for the trailer wiring? Any other idea what could be wrong? [Vauxhall tow bar]
Thanks