So far as I'm aware, it's built into the BIP unit.
JohnE will advise.
The following quote is from my write up about modernising BIP electrics in the 'How to...' guide section:
"The only "problem" that I found was that the awning light now operates irrespective of whether or not the EHU is connected. I thought this was a small price to pay and resigned myself to it but when John rang to check on my progress, he explained that the original BIP had its own 12V transformer and when its circuitry detected that mains current was connected it then switched a relay to allow the awning light to operate. As the 12V transformer has now been removed, John has wired the system to operate a relay on the printed circuit board when the caravan is attached to the car. This relay now inhibits the awning light from operating when towing thereby preventing me from breaking the law. Clever, eh?'