Disco 3. Twin electrics to 13 pin - Advice please!

We are about to buy a 2008 Disco 3 which already has twin electrics fitted into the back bumper recess. Would like to convert to 13 pin socket to match new 'van but professional advice seems to suggest this is not as straightforward as common sense would indicate. I would like to avoid having to use a 'Y' piece connector between the car and 'van. Does anyone out there have any experience of doing this conversion please?