just re read ur post,
i would put my money on it being for the stalk mate,
has it had any headunits in it before??
i know sony headunits connect to the stalk via a jack, previous owner may have fitted the adaptor,
reading your other posts i have a theory,
a previous owner may have invested in a headunit and wanted the stalk to work,
so they invested in a adaptor, either they were too complicated a fit to take out, or they forgot about it.
when they came to sell the car, they ripped the old headunit out and shuved the oem one in there,
would explain the plug, and why your stalk doesnt work with the oem headunit,
was it plugged into anything in the standard unit? if so my theorys out the water