I had one fitted when I had a caravan. The fitter routed the cable from the battery positive, with an in line fuse, across the engine bay attaching it to the vacuum hose running across the scuttle to the brake servo. The cable was then routed through the scuttle with the wiring loom, down past the fuse box in the drivers foot well, then tucked under the carpet down the drivers side, through to the passenger side and round the plastic cover for the wheel arch. The O/S rear cover was removed and the relay fitted in the space in the body behind and below the interior boot light. Connections were then made to the 12s socket. Note that the relay switching point is adjustable, so the relay should switch when the engine is running from the higher voltage generated by the alternator, but should open when the engine is stopped. This is to prevent draining the car battery when the caravan is connected but the car is stationary. Eric.