The Bose "upgrade" includes a 4 channel amp in the boot, whereas the standard system only has the rear speakers seperately amped, so in order to hook up an aftermarket HU you need to know which one you've got, although why they don't just supply the Bose one which would work for non-Bose as well. If you haven't got Bose then it is advisable to bypass the factory amp and rewire the rear speakers directly to the new HU as it is superior quality and more powerful. In this case you don't need an adapter (unless you also have a factory sub), so depends on what car you have.