Hello Karl-Ivar,
Yes, SVXs in Europe use Subaru's older style of radio harness (which consists of two plugs). The third plug (with 2 pins) is for illumination on an in-dash CD player unit. Picture:
93-Legacy-Harness.jpg
Pin-out is here:
http://www.surrealmirage.com/vrg3/bcstereo/ (factory wire colors shown there are different than on European SVX).
I have the correct installation harness for that:
(click to see larger picture)
Pictures of the European SVX radio "cage" and pin-out table with SVX factory wire colors are in this thread:
SVX / Legacy radio compatibility
ADDED: If the third plug you are asking about looks like this...
(click each thumbnail to see a larger picture)
...that connects to the CD changer in the trunk.