What's in it at the moment? My s3 has a Clarion unit I think.
I'll check tonight. If my history file is correct the unit in it is the OEM but the PO had an extra amp installed to power the 2 speakers in the parcel shelf. My car is an American import so that may make a difference too.

you could try asking here -> www.alfabb.com if you need to know what the original fitting was. They are very knowledgeable.

JaB