Does trailer plug need to be coded?

[Devian_NZ]

The B6 came fitted with a towbar and trailer plug, but every time I've hired a trailer, none of the lights work. I've checked the wiring, and it all looks sound (no corrosion, good connections), and it's wired into the loom inside the right-rear cubbyhole, via a little box thing. Would I be right in assuming this is some sort of adaptor that's required to work with the VAGCOM? If so, is it possible that it's lost it's coding some time when they've reset something or worked on the car?