You're supposed to get that before you buy them. Makes paying so much for headlights a little easier, so to speak...
Seriously, I thought it was for the rubber thing that goes over the connector, so it seals?
Current fleet: '85 Ninety 3.9 V8i Auto SW, two Discovery 1's (one for my Nephew, one for my Niece),'07 BMW 335Xi.
Past Land Rovers: '96 Discovery SD, '00 Range Rover HSE, '04 Discovery SE7, '07 Range Rover HSE, '08 LR3 SE7.
Past fleet: '89 Classic Mini, '83 Porsche 911 SC Coupe, '89 911 Carrera targa, '96 911 Carrera 4, '07 Toyota FJ Cruiser, several Minis.