No empty slots, but there isn't suppose to be. Not to be an ass but this may be user error. The PIE unit had a Y cable that plugs into the back of the head unit. Then the plug that USED to go into the head unit plugs into the Y adapter.
Here is the install diagram for the PIE unit:
http://shop.store.yahoo.com/logjam/toy03auxdia.html
The USA spec is probably very similar.
Here is the unit at Crutchfield:
http://akamaipix.crutchfield.com/pro...PA11TOY-f.jpeg