Welcome to NICO! You didn't mention the model year, but I think the part is the same over all years.
Yes, there should be a separate amp. The first picture shows the part and mounting bracket.
As the previous owner replaced the stock HU, I'd be willing to bet he had a different amp installed, but removed it before getting rid of the car. Take it to a local car audio specialist and they should be able to get it sorted out for you.