13 pin towbar connector plug ?

I have an E34 with a factory towbar / trailer socket fitted but I can't find a (local) source of a suitable plug to put on my lightboard cable.
Is this a dealer item only or can someone suggest a source / cost, or is
there an adaptor from 7 to 13 pin ?
Thanks,
Nick
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http://www.towsure.com/default.asp?t 4
on my Audi (factory fit) I use the lead adapter second from bottom
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