John, with post 98 vans there is a habitation relay in the van. It is operated by the fridge supply from the car which is pin 6(+) and pin 7 (-) of the 12S cable. When the relay operates it switches the 12v system in the van off ( causing the lights to go off ) and swithes the permanent feed on pin 4 (+) and 3 (-) to charge the van battery.
The reason your lights are flashing on and off is because there is a problem with the fridge supply, propably the live on pin 6. When the engine starts the supply is there which switches the lights off but the fridge load causes the voltage to drop which releases the relay which allows the lights back on. This is caused by either a high resistance in the supply to the voltage sensitive car relay, or if the relay has an adjuster it needs to be set. To do this start the car engine and with the van connected adjust it so that the van lights go out and stay out but not tooo far. The relays are also notorious for hanging on when the engine is stopped preventing the van lights from coming on and the 12S cable has to be unplugged.
Hope this helps, any questions then please come back to me. Ray